How to replace a piston on a 150cc scooter?

How to tell if replacement is needed

It is clear that the circuit must be changed when there is obvious dysfunction. When the chain wears out a lot, it makes a loud noise. Well, or jumped over the tooth of a star. Or it fell off, which indicates its stretching and wear.

Standard chain lifespans should be close to the lifespan of the scooter engine itself. It's logical. Each owner operates the vehicle differently, but the average chain lifespan is somewhere between 15,000-18,000 km. In a situation where the engine was halved to eliminate other faults, and the chain was immediately replaced after using it for, for example, 9000 km, everything is clear.

There are options for shortening the chain when stretched. But there are some caveats: if the step does not match, the star begins to erase, i.e. the stretched chain eats the camshaft sprocket. If the wear on the star is small, you can isolate the link and ride it a little. You need to do this correctly, adjusting the chain tension with a tensioner, and not forcibly. Otherwise, you will completely damage the chain.

What is typical for a worn chain: A small point: to check, you need to remove it.
The degree of wear can be assessed by the angle of horizontal deflection. If it sags downwards, going almost vertically, change it 100%. The new chain deviates approximately 45 degrees from horizontal. And by laying out the old and new chains on the surface, you will see that the first one will be longer. Also, the wear of the scooter's timing chain can be indirectly judged by the level of extension of the tensioner rod. Critical wear is when the rod extends completely.

How to replace the timing chain in an engine

Having decided on such an action as replacing the timing chain of a scooter, you need to decide on how to implement your plan.

There are two replacement options:

How to change, everyone decides for themselves. Both methods, advantages and disadvantages are described in detail below.

With disassembly

You should choose the right tools - generator and variator pullers (you can make them yourself), a kickstarter sprocket puller, 8, 10, 12 and 17 mm sockets, a hammer, two screwdrivers (flat, Phillips).

The engine is removed from its place, the oil is poured out, all parts are wiped and cleaned. Next, remove the muffler and cooling casing. We remove the generator only with a puller. To do this, unscrew the bolts from the impeller and remove it from the rotor. It is necessary to fix the rotor with a puller and unscrew the nut. Then we tear the rotor off the crankshaft journal. Remove the variator cover. Also, secure the impeller with a puller, unscrew the nut, and remove the variator from the axle.

Below the variator is the kickstarter gear. Likewise, a puller is required for it. Then we clean the cylinder connector, valve covers and cylinder head. After unscrewing the bolts on the valve covers, remove it. Next, remove the scooter's timing chain tensioner. After unscrewing the bolts on the cylinder head, we pull out the camshaft bed.

Next, remove the chain from the camshaft and pull it out of bed. To avoid losing the guide bushings that center the parts relative to each other, you need to remove them and put them away. We remove the cylinder head, the cylinder itself, and the piston (to do this, remove the retaining ring and pin from the upper connecting rod head).

After unscrewing the bolts on the crankcase cover, tapping it a little, remove it from the engine. We take out the starter intermediate gear, use a screwdriver and a hammer to unscrew the overrunning clutch nut (clockwise) and remove the gear and housing from the trunnion. We reached the chain!

Remove the protective shield from the oil pump. Place a metal stick under the chain, twist the nut on the oil pump sprocket, remove it and remove the chain.

No disassembly

We take the following tools: a Phillips screwdriver, pliers, a new chain and a special puller. More details about it will be below. We flare the new chain - to do this, use a puller to press the pin out of the joint of the links, but not completely.

A little about the puller

It is called a timing chain puller. It can be made by a turner by turning a threaded extruding pin and rolling a strong pin into it with a diameter similar to that of a chain pin. You can purchase a ready-made bicycle one. Just narrow the pin a little to the desired size.

Remove the cooling casing from the scooter engine, then the cylinder head cover. Next you need to remove the chain tensioner. We remove the camshaft bed, take out the shaft, remove the star.

We take the puller, center it with the pin, and gradually press out the pin. You can press it out completely, it will not be useful. We compare the links of the new and old chains, thread the wire into the seat of the pin and fix the ends of both chains. We turn the cooling impeller counterclockwise, gradually replacing the old chain with a new one.

Then we separate the ends of the chains, throw out the old one, and combine the new links from the ends. We check the alignment, install the pin, and use pliers to carefully press the pin in so as not to damage the new chain. We first check the mobility of the links. We finalize the pin with a puller. Again we control mobility. This point is very important: if you install the pin unevenly and miss it, it will begin to rest against one of the links and squeeze out the next one.

As a result, the subsequent pin or link bends, causing it to jam and become immobile.

Next, we assemble the engine in reverse order, placing marks on the rotor. When installing the camshaft bed, after tightening the nuts, you need to check the valve clearances with a feeler gauge. Upper valve – inlet: 0.05mm, lower – exhaust: – 0.1. The feeler gauge should fit freely between the valve and the adjusting bolt.

Then install the chain tensioner. We unscrew the bolt on the top of the tensioner, install a screwdriver in the groove, and, turning it, push the rod back. We fix everything else back according to the diagram.

Set timing marks to 139qmb

1. Install a new cylinder head gasket.

ATTENTION!

The cylinder head gasket is made of soft metal (copper, aluminum) and after tightening the cylinder head nuts it is deformed, thereby ensuring a tight connection. Reinstallation of the cylinder head gasket is not permitted.

2. Install the timing chain guide.

3. Install the cylinder head.

4. Set the piston to TDC according to the marks on the flywheel. This is usually the "T" mark on the flywheel. Align the mark with the tide on the engine crankcase.

ATTENTION!

The following operation is one of the most important when assembling the engine. It is important to align all the marks very accurately. An error in installing the chain even by 1-2 teeth will result in the engine not working correctly, and it will probably even lead to serious damage to the timing and cylinder-piston gas parts when trying to start the engine!

5. After making sure that the timing chain has not jumped off the drive sprocket on the crankshaft, install the camshaft into the cylinder head, at the same time putting a chain on its sprocket so that the marks on the camshaft sprocket are parallel to the plane of the edge of the cylinder head, and the cams are directed downward (inside the head).

6. Install the rocker arm support, orienting it according to the installation marks. Lubricate the cams and camshaft bearings.

ATTENTION!

ATTENTION!

If the copper washers are severely deformed (flattened), they must be replaced, since if the washers do not fit tightly due to oil leakage, the pressure in the lubrication system will drop and the motor will quickly fail. Sealing this connection with any sealant is unacceptable.

7. Install sealing copper washers on the studs.

8. Screw on all four cylinder head nuts without tightening them.

9. We recess the working rod of the timing chain tensioner inside the housing.

10. Reinstall the timing chain tensioner and tighten its mounting bolts.

11. Insert the working bolt and the timing chain tensioner spring, tighten it. In this case, you can hear characteristic clicks - the ratcheting mechanism of the tensioner working rod will be activated, which, under the influence of a spring, will come out of the housing and tension the timing chain.

13. Screw in and tighten the additional cylinder head bolts.

14. Using a socket wrench for the central bolt of the flywheel, or with your hands holding the flywheel, slowly turn the engine crankshaft clockwise 3-4 turns to make sure that the timing timing is set correctly and the piston does not collide with the valves.

15. We install the crankshaft at TDC of the compression stroke and once again check the correct installation of the camshaft according to the marks.

17. Rotate the crankshaft several turns and once again check that the valve clearances are set correctly.

18. Install the cylinder head cover and tighten its bolts.

19. Place the crankcase ventilation hose onto the fitting in the cylinder head cover.

20. Install the cylinder cooling casing.

ATTENTION!

Do not forget to remove the paper from the intake manifold if you covered it from dirt during disassembly.

21. Install the inlet pipe assembly with the carburetor.

ATTENTION!

In most cases, when installing an exhaust system, the exhaust pipe gasket will need to be replaced. It is better to replace it with a new one during assembly.

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Which method is preferable?

It is logical that the first option with complete disassembly of the scooter engine is more complicated and takes more time and effort. But, as you can see, it is more reliable. Firstly, the factory integrity of the circuit is not compromised; secondly, when disassembling the scooter engine, you can see any defects, such as cracks, wear, change the seals, clean the parts.

The second option is more simplified and can be used when you only need to replace the chain. And you are confident in the quality of your work.

Final word

The scooter is a fairly popular vehicle and is in increasing demand. Because it leaves traffic jams behind, has low operating costs and is economical in terms of fuel and lubricants. Many people prefer to do scooter repairs on their own, especially since four-stroke engines are simple and reliable to use. Timely technical inspections, careful operation and the above article will allow you to enjoy riding this miracle of the Asian motorcycle industry for a long time.

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Dismantling the piston

If you do not have experience in repairing a scooter, it is better to entrust the replacement of the piston on a 150cc scooter to professionals. Otherwise, you may make a mistake, which will lead to serious damage. How to change the piston on a 150cc scooter? You need to perform several sequential steps.

Dismantling and installation of piston rings on a 150cc scooter and other components of the unit is performed in the same way for all two-wheelers of the presented class. If the engine is 4-stroke, you need to do the following:

• The plastic protecting the motor is removed.
• The carburetor is removed. There is no need to disconnect the hoses from it.
• At the point where the muffler is attached to the cylinder head, the fastener is unscrewed. Unscrew the 2 bolts holding the engine crankcase. Next you can remove the muffler.
• The plastic cover of the cylinder head is unscrewed. After unscrewing the 4 bolts, it needs to be removed.
• Near the carburetor there is a chain tensioner, which also needs to be unscrewed.
• The 4 bolts holding the cylinder head are unscrewed, and the head itself is removed.
• The cylinder is removed from the studs.

When dismantling the piston on a 150cc scooter, you need to fix the piston at the top point (you need to follow the marks on the generator rotor). It is important that the chain does not fly off the sprocket. Otherwise, installing the drive will take a long time when assembling the piston in reverse order on a 150 cc scooter.

The retaining rings are removed from the piston and the pin is removed. At this point, the piston group of the scooter 150 is considered dismantled.

Replacing the timing chain on a 139qmb engine the easy way

How I replaced the timing chain on a 139qmb engine of a Chinese scooter, and at the same time checked the possibility of shortening the timing chain

When I first opened the valve cover on the 139qmb engine, shortly after purchasing a used scooter, it was discovered that the timing chain was stretched to the limit and even more. The tensioner was pulled all the way out, but the chain was still loose. with spare parts for scooters in our area, and attempts to find them in stores led to nothing. I've been driving like this so far. But soon there were two instances of the chain slipping on the camshaft or crankshaft sprocket, but in any case in such a situation, having turned off the engine, it was impossible to start it again until the camshaft was installed according to the marks. Since there is still no timing chain Under the cylinder from thick (2 mm) cardboard, under the head - from metal asbestos, also about 2 mm thick. The chain no longer jumped on the teeth, but it was still rather weak. You won't be able to travel like this for long. There was an idea - to shorten the chain by throwing out one link, but then on one site I found an acceptable option for me to purchase a chain through an online store by mail with cash on delivery - honlingzakaz.ru - Timing chain 50cc (139QMB). Simple registration, the only thing is that the minimum order price was no less than 400 rubles and in addition I ordered head and cylinder gaskets. Placed an order by registering on the website (you need a valid e-mail and phone number). After some time they contacted me and I confirmed the order. A week later I received the parcel at the post office.

Proper maintenance and adjustment of the motorcycle timing chain

Modern engines of imported motorcycles require almost no attention, except for timely oil and filter changes. But on motorcycles with a fairly high mileage, there still comes a moment when the engine itself suggests with increased noise that it is time to start servicing the timing system - gas distribution mechanism. We will look at how to do this correctly without harming your engine in this article. Unlike modern car engines (ancient Ladas and Muscovites do not count), (and Italian exclusives, such as Ducati motorcycles), where the gas distribution mechanism is controlled by a rubber gear belt; on most modern imported motorcycles, the timing chain commands the timing chain. Of course, there are exceptions on some large cruisers, or English classic motorcycles, which use camshaft drive using gears. But such a drive does not need adjustment at all, since if after a very long mileage parts wear out, they are simply replaced with new ones (the same gears).

With a chain, everything is not as simple as it seems, since it must be constantly tensioned with a certain force in order to accurately transmit torque and serve the required period. After all, the camshaft of any engine must rotate absolutely synchronously with the crankshaft.

And if there is a slight lag or advance even by 1 -1.5 °, then the engine of your motorcycle will lose its power, and the wear of timing parts will increase. If the lag or advance increases by several degrees, then it is quite likely that the pistons will meet the valves, and this is already very serious. In most cases, many parts of the gas distribution mechanism will be destroyed, and most likely it will be easier to replace the engine head completely. I think many have understood the importance of the chain tension system, now it remains to understand all this in more detail.

To tension the chain with a certain force, a set of parts intended for this purpose is used, consisting in most cases of two dampers coated with anti-friction material, along which the chain slides during operation, and the chain tensioner itself.

The tensioner is mounted in the engine so as to rest against the driven arm of the chain, and this arm is constantly pressed with the required force. This entire unit (and its normal operation) is very important, since a malfunction of this unit is dangerous and can lead to the dire consequences described above. And although problems here are quite rare, it still deserves attention, since in order to correctly diagnose the tensioner parts and then assemble them correctly, you need to know well how this unit works. Semi-automatic mechanical tensioner. Tensioner designs, depending on the years of manufacture, vary some. On the most ancient models until the 80s, simple mechanical timing tensioners were installed (see photo on the left), which were very easy to set up.

All you had to do was loosen the tightening bolt (usually a 10mm wrench), then rotate the engine crankshaft as it rotates until the resistance weakens, and re-tighten the bolt. Everything is quite fast and simple.

But after 1980, tensioners began to appear that automatically tighten the chain relative to its stretch. They are still used today. Some automatic tensioner systems are slightly different from each other, but their essence is the same - if they once extend to the required distance, the tensioner rod is fixed in this position and cannot move back. Thanks to this, the timing chain is constantly tensioned at all times with a certain force, which is set by the tensioner spring.

But many will wonder: why bother with the tension system if everything is automated.

Well, first of all, I already said at the very beginning of the article that you only have to climb into this unit on motorcycles with high mileage, because after a certain mileage everything eventually wears out. And secondly, when disassembling the engine (for example, to repair it), or replacing tensioner parts, many novice repairmen make mistakes, which in the end can turn out to be very expensive. My goal, or rather the goal of this article, is to warn against these mistakes. One of the common mistakes of novice “masters” is when, when removing the valve cover of some engines, on which the damper is located, the chain naturally weakens, since there is no longer pressure on the damper, the cover is then removed .

And when the chain tension is weakened, a working tensioner will immediately tighten the chain by extending slightly. A novice “master”, having checked the chain tension, will naturally be pleased with the normal tension. Only now he will not be able to install the valve cover in place normally, since the damper located in the cover will rest against the tense chain.

The unfortunate mechanic begins to press harder on the cover and compress it with standard bolts. The chain guide into which the tensioner rod rests is slightly worn. If the chain is tightly tensioned, it can wear out in a few minutes of work. As a result, either the tensioner clamp or the tensioner shoe itself (or the tensioner) breaks. If the high-quality parts of the tensioner and stabilizer can withstand such abuse, then the next time the engine starts, the chain will be so tense that the friction material of the tensioner and stabilizer will be erased in a few minutes of operation.

And the chain itself, during the same short operating time, will stretch as if it had a long mileage. As a result, the wear and noise of this unit will increase many times over, and very quickly. Therefore, before assembling the unit, I advise you to study the principle of operation of the tensioner of your particular engine, and find a way to return the tensioner rod back.

Below I will tell you how to do this correctly. The sprocket of a modern silent chain looks like a gear. 1 - sprocket, 2 - additional gear of the motor transmission, 3 - main gear, 4 - damper springs. As a rule, with high mileage, the timing chain stretches and begins to ring. In most cases, eliminating this defect requires replacing the chain, and the dampers too. But many novice “mechanics”, placing one or more washers under the tensioner spring, think that they have solved the problem. But this does not help for very long, or does not help at all. After all, the sprocket of a modern silent chain looks like a gear. A modern silent chain (see photo) is designed in such a way that, unlike a conventional chain, it covers a tooth with a much larger link area.

When any chain is pulled out, increased gaps appear between the sprocket tooth and the link, which cannot be corrected by anything, the link is stretched, and the distance between the axes (pins) of the link increases, wear is wear. This causes increased noise, but that’s half the problem.

If you add all the increased clearances of each chain link together, you get a significant lag between the camshaft and the crankshaft. The engine loses some of its power because the valve timing begins to lag. For information: if the chain has 114 - 116 hinges (links) - depending on the type of engine, then the wear of each of these hinges by only 0.08 mm, as if adding another link to the total length of the chain, and from this the timing sprockets, ignition switches are shifted back in phase from the sprocket on the crankshaft, by an entire chain link! As a result, the power characteristics and economic indicators of the engine are significantly reduced. Now imagine that if the links wear out not by 0.08 mm, but more, how will the engine start to work? The conclusion suggests itself - an elongated motor chain will no longer be able to transmit rotation normally and in a timely manner from the crankshaft to the camshaft. A modern silent motor chain, the links of which almost completely cover the sprocket teeth.

From everything written above, let us understand for ourselves that only a new chain can return the engine to its former strength, and if the friction material of the tensioner and damper is worn out, then their replacement is also extremely necessary.

Inspect the chain sprockets; there should be no signs of wear visible to the naked eye. And if, with the tensioner mechanism assembled and the chain in place, the rod of your tensioner is fully extended, then the motor chain of your bike is completely stretched, and it’s time to replace it with a new one. To replace the chain without removing the sprockets, you need to unpress it, and then stretch a new chain and press it using a special tool. How to properly stretch a new chain, and about the device itself for pressing the chain, I advise you to read this article. Still, let’s look at some types of tensioners in a little more detail, since I think this will be useful for novice mechanics.

Rack tensioner. 1 - spring plug screw, 2 - tensioner housing, 3 - spring-loaded ratchet rack stopper, 4 - rack itself, 5 - spring, 6 - plug sealing washer. The red arrow shows where to press to pull out the latch and then recess the pusher rod.

Rack tensioner . This is one of the ancient mechanical tensioners, but it already works automatically (and not by adjusting it with a key, as I described above). It is quite reliable and does not require attention for quite a long time. The main working element of this type of tensioner is the pusher. It is constantly pressed by a spring in the direction of tensioning the chain, and an ordinary ratchet mechanism does not allow the pusher to return in the opposite direction (yes there, no back). The ratchet mechanism is very simple and is a spring-loaded stopper resting on a toothed rack. After removing the tensioner from the engine, the pusher on a working unit should move out completely, and if you try to push it even more with your hands, it will come out again, which means the spring has sagged and needs to be replaced.

Well, if, when trying to push the pusher inward, on the contrary, you managed to do this, then the ratchet mechanism of the ratchet is faulty (the teeth are broken or worn out). Naturally, you need to replace such a part with a new one.

Having pressed the lock with your finger, you can now push the pusher rod inward. Before installing such a tensioner back on the engine, inspect all parts for wear, and if there are no visible signs of wear and jamming of the mechanism, then you can install it back on the engine. To do this, you need to unscrew the spring plug 1 from the tensioner body (see photo above), and then by pulling the spring-loaded stopper away from the teeth of the rack (for example, with a thin screwdriver or with your finger as in the photo on the left), you can now push the pusher back (since you pull the lock with a screwdriver or a finger and it doesn’t interfere). Well, after you secure the pusher itself to your engine with screws, you can install the spring back and compress it with the screw plug 1.

Screw timing chain tensioner. 1 — bolt plug, 2 — screw tensioner body, 3 — the pusher itself.

Screw tensioner . On more modern motorcycle engines, a screw tensioner is installed, which, unlike the rack-and-pinion tensioner described above, regulates the spring tension more smoothly due to the rotation of the screw, which smoothly extends the pusher. And the thread of a special screw does not allow the pusher to slide back into the body. You can check the proper operation of such a tensioner as follows. Having unscrewed the screw plug from the pusher body, and then inserted a thin screwdriver into the body and felt the screw with it, turn it, and the pusher will begin to move into the body (the pusher should not move in or out with your hands, the pusher play should not be felt either). Then try to loosen the screw (remove the screwdriver), and at this moment the pusher should quickly move out with a click until it stops. When rotating the screw with a screwdriver, you should feel only the tension of the compressed spring, and the screw itself should rotate easily and without jamming. Checking the functionality of the screw tensioner. In the same way, such a tensioner is installed back on the engine, that is, we turn the screw with a screwdriver until the pusher is completely seated body, then holding the screw in this position with a screwdriver, with the help of an assistant, try to install the tensioner in place on the motor, tighten the bolts securing it, and only then release the screwdriver. All that remains is to tighten the plug screw. Wedge timing chain tensioner. 1 — wedge spring cover-plug, 2 — tensioner body, 3 — wedge, 4 — wedge spring, 5 — pusher spring, 6 — limiter bolt, 7 — groove for the limiter bolt, 8 — pusher. The red ring indicates a broken lid. Wedge tensioner . A motor timing chain tensioner with a wedge tensioner is less commonly installed on motorcycle engines, but it is still worth writing about. This tensioner is not as compact as the mechanisms described above, but it is impossible to break it, since it has neither small teeth nor fine and delicate threads. The tension of the motor chain is also smoothly adjustable, just like on a screw tensioner.

The pusher itself is pushed out by a spring, and is secured from unnecessary movement by a special bolt that presses on the groove in the pusher (see photo). And the pusher is prevented from returning back into the body by a wedge, constantly pressed by a spring.

After inspecting the tensioner parts for professional suitability, and not finding visible signs of wear, pusher play or jamming, you can return the tensioner back to the engine. But before this, it is necessary to recess the pusher, and to do this, unscrew the spring support cover, then remove the spring, and only after that can you remove the wedge that prevents the tensioner rod from moving back into the body. Having installed the chain tensioner in its place on the engine, now calmly insert the wedge and spring into place, and then screw on the cap, which will tighten the spring and wedge.

Hydraulic motor timing chain tensioner mounted on the engine. A tube from the oil system of this engine is connected to it.

Hydraulic tensioner . They are also called oil tensioners, since they work from the pressure of oil entering the inside of the hydraulic cylinder from the engine lubrication system. Such tensioners have long been used on many cars, but they began to be used on motorcycles in recent years, and even then not so often. They are mainly used on quiet engines, since on forced engines the oil pressure can vary widely, and this will cause the chain tension to change, naturally this is not good. Moreover, for the normal and smooth operation of such a system, the engine must Very clean and high-quality engine oil is always present, since inside the tensioner there is a plunger manufactured with high precision and with a very small clearance in the hydraulic tensioner cylinder.

And if even the slightest speck gets into this pair (plunger and cylinder) with oil, then the plunger with the tensioner rod will simply jam. The consequences are very sad, since no matter how much oil pressure is in the system, it will no longer be able to push out the jammed plunger and chain tensioner rod. This will weaken the chain and the timing will no longer operate normally.

Therefore, the purity of the oil and its timely replacement play a huge role here. The hydraulic timing chain tensioner is removed from the engine. The polished pusher rod is visible. Moreover, in cold weather, or after a long stay (when the oil drains into the sump), engines with a hydraulic tensioner rattle the chain for several seconds of engine operation (until the oil fills the hydraulic cylinder to the required pressure). At this point, the main thing is not to increase the speed of your engine until the sound goes away. However, I do not recommend increasing the speed on any engine until it warms up. Two automatic tensioners (rack and pinion on the right), which must be cocked before installation on the engine.

There is also an automatic tensioner, as in the photo on the left, in which, before installing it on the engine, you just need to cock the spring-loaded handwheel (turn the wheel) and at the same time sink the tensioner pusher rod. After installing such a tensioner on your engine, turn the wheel back, and the pusher rod will move out freely and tension the chain.

Remember that if, on any type of tensioner, the pushrod is pushed outward as much as possible, then your bike's timing chain is completely stretched (worn out). Wear of parts such as friction material (slides) of dampers is usually visible visually. Naturally worn or loose parts must be replaced with new ones. We cock this tensioner by turning the wheel and at the same time pressing on the pusher rod. If you repaired the engine (you can read about the correct repair of a Japanese four-stroke engine here), then after assembling it, install the motor chain tensioner last, and as described above - with a completely recessed pusher.

After installing the tensioner on the engine, rotate the crankshaft of the assembled engine 4-5 revolutions in the direction of rotation (by the crankshaft bolt), and at the same time make sure that the tensioner friction shoe takes its working position.

Don’t even think about performing the same operation, rotating the camshaft instead of the crankshaft. I hope now you will be able to more competently service the timing belt of an imported motorcycle, and most importantly, diagnose this unit and modify the parts yourself, because high-quality motorcycle service, especially in the outback, requires just dream for now. Good luck everyone!.

Photo report: How to check the timing chain of a scooter?

It’s always like this in life: one thing is good, the other is bad. Exactly the same principle applies to various types of mechanical torque transmissions. Chain transmission was no exception here. On the one hand, there is nothing simpler, more reliable, lighter, more compact and cheaper than a chain drive.

On the other hand, we get eternal “hemorrhage” with stretching of the chain, wear of the sprockets, endless cleaning and lubrication, maintenance and tension of this very chain. But there is nothing to be done, the chain transmission has a lot of advantages over other types of mechanical transmissions, so these types of transmissions are used almost everywhere.

However, today we will not talk about the chain drive as such, but about a method for diagnosing the timing chain driving the camshaft of a scooter engine.

To change or not to change?

Suppose you “overhaul” your engine: you change the seals, crankshaft, gaskets and other devices, but you doubt whether to replace the timing chain with a new one or not. If you doubt it in vain, change it for a new one at the first opportunity. And now I will explain why.

See for yourself: a chain has an average service life of approximately 18,000-20,000 km of a scooter (personal experience), approximately the same as the average service life of the engine itself. And for example, after driving 6,000 km, the crankshaft jammed or the engine leaked, or something else happened that requires complete disassembly of the engine. Naturally, you disassemble the engine, replace the crankshaft with a new one, and leave the chain, which by that time has exhausted a third of its service life.

And what happens? The chain, after 12,000 km, will exhaust its service life and become unusable, but the crankshaft and other parts will be in good order and you, like it or not, will have to again, because of the chain alone, half the engine. Is it necessary? The chain is a consumable item. Never skimp on it!

Examination

Personally, I never check timing chains. I simply throw them in the trash, no matter how awesome they seem to me. For me, a chain is a consumable, just like a spark plug, oil or oil seal. But if you are important, or money is tight, you can use the old fashioned way.

We stretch the chain along its length, hold it with our fingers and try to position it in a horizontal position

For clarity, I took a new and heavily worn chain, put them together and checked for horizontal deflection. As you can see: the new chain (above) described a barely noticeable arc, while the worn and useless chain sagged like snot, almost at an angle of 90 degrees.

The worn chain turned out to be almost a centimeter longer compared to the new one.

How to replace the timing chain of a scooter engine?

It’s very simple, my friends: you remove the engine from the frame, disassemble it and replace the chain with a new one. There are no special secrets in this matter, just as there are no special ways to “cheat fate” and replace the chain without disassembling the engine. The chain can only be replaced by completely disassembling the engine.

The timing chain is in no way designed to be riveted or riveted. During the riveting process, it can easily be pulled or twisted, and this is a sure way for the chain to fly off the sprocket while the engine is running, and then, depending on your luck... Maybe everything will work out, or maybe the valves will bend. And one more point: a beginner will not be able to rivet a chain efficiently. This work is not as simple as it might seem at first glance. Therefore, don’t suffer from bullshit and disassemble the engine.

How to adjust and tension the chain?

There’s no way you can tighten it or adjust it, and you don’t bother Google or Yandex search with such queries. The chain in the scooter engine is tensioned automatically. And if it has already stretched to such an extent that it begins to fly off or jump over the teeth, feel free to disassemble the engine and replace it with a new one. There are no miracles.

And I will prove this to you now. The chain in the engine is tensioned using a special tensioner, which operates in a fully automatic mode. As the chain stretches, the tensioner rod, under the action of an internal spring, comes out of the housing and tightens the chain. And the more the chain stretches, the more the rod will come out. But! The rod has a limited stroke: on a completely new chain it comes out about a third of its stroke and then, as it stretches, it comes out completely and then it won’t come out anymore. Therefore, the chain will no longer be tensioned! And all your attempts to correct the situation will lead to nothing.

To change or not to change?

Suppose you “overhaul” your engine: you change the seals, crankshaft, gaskets and other devices, but you doubt whether to replace the timing chain with a new one or not. If you doubt it in vain, change it for a new one at the first opportunity. And now I will explain why.

See for yourself: a chain has an average service life of approximately 18,000-20,000 km of a scooter (personal experience), approximately the same as the average service life of the engine itself. And for example, after driving 6,000 km, the crankshaft jammed or the engine leaked, or something else happened that requires complete disassembly of the engine. Naturally, you disassemble the engine, replace the crankshaft with a new one, and leave the chain, which by that time has exhausted a third of its service life.

And what happens? The chain, after 12,000 km, will exhaust its service life and become unusable, but the crankshaft and other parts will be in good order and you, like it or not, will have to again, because of the chain alone, half the engine. Is it necessary? The chain is a consumable item. Never skimp on it!

Additional actions

If you know how to tighten a motorcycle chain, it wouldn't hurt to check the condition of the sprockets and other transmission parts. If signs of significant wear are detected, they should be replaced immediately, since breakdowns of this type seriously affect the reliability and durability of the motorcycle transmission. If you find that the chain is very stretched, it is better to get rid of it immediately - especially if you like to ride at high speed or perform stunts by tipping the motorcycle onto its rear wheel.

In addition, when regularly tightening the chain, it should also be cleaned and lubricated. It is better to use special products in cans that are supplied under high pressure - they effectively wash away all foreign deposits and also penetrate into all internal parts of the chain drive, contributing to its effective lubrication. The procedures are also performed with the rear of the motorcycle raised, which allows you to quickly spin the wheel if necessary. There are also special devices for cleaning the chain. The device is fixed to the transmission mechanism of the vehicle, after which all that remains is to engage first gear and ensure a continuous supply of cleaning agent to the receiving tube.

The motorcycle is serviced approximately every 500–2000 kilometers, which ensures the reliability and durability of all mechanisms. Every time you clean and lubricate, you should check the degree of chain tension to prevent serious sagging. Experts say that on average, tensioning has to be done every 2–5 thousand kilometers, depending on the operating conditions of the motorcycle. It is worth completely changing the chain every 10–15 thousand kilometers, or more often with an aggressive driving style or with frequent off-road trips. Failure to comply with these deadlines leads to accelerated wear of the transmission and the approach of expensive motorcycle repairs.

Examination

First, you should determine whether your chain needs tension - for this you need to take a control measurement. The motorcycle must be placed on the side stand, removing panniers and other foreign objects from it. As a tool, you should take a tape measure, a ruler or a fairly large caliper. Make sure there is no load on the suspension and start taking measurements.

Very often the question arises, how should the chain be tensioned to avoid accelerated wear of motorcycle components? It is best to check the operating instructions developed by the motorcycle manufacturer, since the standard indicator may be individual for a particular vehicle. If this is not possible, you should use average values ​​equal to:

When checking, it may be discovered that the actual indicator is outside the acceptable range. In this case, it is necessary to urgently tension the chain to avoid motorcycle breakdown in the near future. In addition, it is worth performing another check, assessing the technical condition of the circuit. Gradually spinning the rear wheel, determine the degree of sagging at each point - if this indicator differs significantly, the part urgently needs to be replaced. If your chain is very stretched, but you need to get to a store or service center, then to adjust you need to choose a position with minimal slack.

How to tighten the chain on an Alpha moped?

A distinctive feature of a moped from a scooter is that the former has a chain instead of a belt, so sooner or later quite typical breakdowns occur with Alpha mopeds. The chain tends to stretch over time, after which movement becomes quite problematic. You can tension the chain yourself; no special skills are required. But know that you don’t need to tighten the chain on the Alpha moped too much; the free play should be from 2 to 3 cm, otherwise you will face much bigger problems.

So, if you don’t know how to tighten the chain on an alpha moped, follow the instructions below.

First of all, remove the passenger footrest so that it does not interfere with tensioning. Then place the alpha moped on the center stand. But some experts recommend tensioning under a certain load on the moped, because on the center stand your vehicle will be supported by weight; after tensioning the chain, you will notice that it has become even more tense, like a string. This oversight leads to serious damage; many say that an overtightened chain after 1 thousand km leads to a breakdown of the entire clutch mechanism.

How to adjust and tension the chain?

There’s no way you can tighten it or adjust it, and you don’t bother Google or Yandex search with such queries. The chain in the scooter engine is tensioned automatically. And if it has already stretched to such an extent that it begins to fly off or jump over the teeth, feel free to disassemble the engine and replace it with a new one. There are no miracles.

And I will prove this to you now. The chain in the engine is tensioned using a special tensioner, which operates in a fully automatic mode. As the chain stretches, the tensioner rod, under the action of an internal spring, comes out of the housing and tightens the chain. And the more the chain stretches, the more the rod will come out. But! The rod has a limited stroke: on a completely new chain it comes out about a third of its stroke and then, as it stretches, it comes out completely and then it won’t come out anymore. Therefore, the chain will no longer be tensioned! And all your attempts to correct the situation will lead to nothing.

Preparatory work

It is imperative to clean the chain, otherwise tensioning will not work. There is grease on metal surfaces, and dust accumulates on top over time. These contaminants cannot be washed off in ordinary water; you need to use auto chemicals. It is better to choose a product in the form of an aerosol that will not corrode the seals - chain O-rings

If the mechanism is heavily dirty, it is advisable to remove the cover from the drive sprocket. It is important to ensure that the retaining nut is well tightened. The cleaning itself is carried out as follows:

Cleaning the moped chain Now you need to determine the degree of wear of the elements. The chain links should not jam, ideally they move easily. It is also necessary to check the presence and condition of the seals. The sprocket teeth must not be bent, pointed or damaged. Wear on the tooth surfaces may be associated with improper installation of the rear wheel.

To determine the condition of the chain, you need to check whether there are areas where the tension is too high or, conversely, weak. To do this, you need to manually turn the wheel. If such areas are present, it means that the bushings and pins have worn unevenly. In this case, the distance between the pins increases and the chain becomes longer.

Severely worn chain

Often the cause of the appearance of sections with different tensions are problems with other components of the moped. The sprockets may be installed with an incorrectly centered hole or the wrong diameter. Uneven tension may be due to severe wear on the wheel hub or failure of the sprocket hub bearings. But other damage may also be discovered, so it is important to carry out a thorough inspection.

It is easy to determine the degree of wear by the length of the chain. To do this you need to pull it back. It is important to try to pull the chain off the sprocket at the same time. Replacement is required when, when attempting to remove, most of the sprocket tooth becomes visible. If the chain rollers are bent and squeak when driving, it is better to change the part along with the sprockets.

After cleaning, you need to align the rear wheel. This is an easy task if the axis is marked. But the marks sometimes get confused, so additional checking is necessary. As the rear wheel rotates, you need to shine a flashlight and pay attention to the chain. If it does not move straight, it means the installation was not done correctly.

After installing the rear wheel, you need to remember where the marks are located. If they are not symmetrical, this indicates that the marks were applied incorrectly. In this case, you can draw new marks on the reverse side of the chain with a marker. When tightening the rear axle, it is important to push the wrench toward the front wheel.

How to tension a chain on a moped?

One distinguishing feature of a moped from a scooter is that a moped has a chain instead of a belt. Therefore, over time it can stretch. You can tension the chain without any skills, it is not difficult at all, so you can do it yourself. It is recommended not to tighten the chain too much; the free play should be from 2 to 3 cm, otherwise the chain may break. Also, do not forget to lubricate the chain with oil. In order to tension the chain you will need: 4 keys, 10, 14, 17 and 24.

How to tension a chain on a moped? Link to the main publication moped-info.ru

How to tension a timing chain on a scooter

how tight should it be?
when should it be changed? FIFTI__50

, Chinese 4t scooters have an automatic chain tensioner. He regulates the tension himself. After removing the tensioner from the engine (and it is necessary to remove it), you need to disassemble it (tighten one large bolt from above, it is better to loosen it before removing it), and pull out the spring.

Then, press the safety tongue from below and lower the gear rod inside the tensioner body. After assembling the piston and tightening the power bolts on the rocker arm, install the disassembled tensioner on the valve head body and insert the spring. Press it down on top with the cap bolt and screw the bolt back in and tighten. When tightening, do not break the aluminum thread.

While the engine is running, the tensioner itself will push the rod as far as it can go. As the chain wears out, it will gradually move further and further. Critical wear - fully extended rod. I have only come across similar cases twice so far. With such wear, the timing chain will one day jump a tooth or several. And it’s good if it doesn’t fly off at full speed.

Of course, there may be cases of strong tension on the chain during installation - when the new chain is strongly tensioned on its own. This usually happens when the parts do not match - non-original piston parts were installed, or the chain is unsuitable. The thick gasket under the pot - with a metal ring inside and rubberized tracks - may also be to blame. Instead, in this case, it is better to find a relief metal gasket. Look like that's it.

Professional approach

Many motorcyclists treat chain tensioning as a routine procedure, during which significant tolerances can be made and the position of the rear wheel can be adjusted “by eye”. This approach when setting the required chain tension is unacceptable - all operations must be performed as carefully and accurately as possible, using special tools. If you want to extend the life of the chain and avoid severe wear on the transmission when using the motorcycle in extreme conditions, you should purchase a special laser gauge to set the optimal wheel position. In the most difficult cases, if you cannot understand the structure of the adjusting mechanism or cannot loosen it, you should contact the specialists of a motorcycle service station. They will carry out all the necessary work, providing you with a guarantee of the quality of the result and the reliability of the motorcycle.

Preparation

So that you don’t have green snot later about the fact that the tuning piston does not fit into the crankcase and you were allegedly deceived here - remove the old piston from the engine and measure the hole for the liner with a caliper. If the hole is smaller than the tuning piston sleeve, the “meat” of the crankcase allows it, hands grow from where it is needed - a round file will help you (only before boring - disassemble the engine, assemble the empty crankcase with bolts, bore the hole, make sure that the piston enters the crankcase without problems - wash the parts from shavings and reassemble the engine. Or, as a last resort, coat the crankcase and file with grease to at least somehow protect yourself from sawdust getting into the inside of the engine.)

The hole for the 162QMK engine sleeve (such engines are installed on Chinese scabs, more popularly known as “Leika”) in stock is 65 mm. Without boring, a piston of 57mm (150CC), 61mm (1700CC) and 62mm (180CC) easily fits into this crankcase. You won’t find a 62mm piston on sale during the day, so it was decided to install a regular 61mm piston on this engine.

By the way, for those who do not know, a piston with a diameter of 61 mm can be easily installed not only on the Leica engine, but also on any other Chinese 150 equipped with a 157QMJ series engine. There, if you put your hands to it, you can stick a piston at 62mm, but only if the “meat” of the crankcase allows it. Everything is individual, you need to look at the place. In any case, the distance along the studs is the same for both the 61mm piston and the 62mm piston.

On the 139QMB series engine from a regular Chinese 80-six, you can easily install a piston 39mm (45CC), 47mm (72CC), 50mm (84CC)

From left to right: 39mm (45CC), 47mm (72CC), 50mm (84CC)

Installing a new node

You need to get a new piston group for a 150cc scooter. Installation is carried out in reverse order. A small amount of oil should be applied to the piston, cylinder and rings. Next, a ring (thin) is put on the piston into the lower groove. Then the oil scraper component and the thin ring are installed again. Next you need to mount 2 compression rings (they are thicker). It is important that the sections of the presented components are not located one above the other.

Now the piston needs to be placed on the connecting rod. The position of the part on the crankshaft is fixed with locking rings and a pin. Bushings are installed in the new cylinder.

The cylinder gasket fits onto the studs. The prepared element can be put on the piston. You must act carefully so as not to damage the cylinder. The chain is pushed into the groove, the chain bar is mounted on the head. The chain is put on the sprocket so that the 2 smaller holes are exactly on the sides, and the large one is at the top. The cylinder head is fixed, the plastic cover is put in its original place. Other parts are assembled in reverse order.

If you want to choose a reliable scooter, contact the employees of our Dvako Moto online store https://dvako.com.ua/. We offer a wide selection of motorcycles. In our online catalog you can order new and used models with a 1-year warranty. Delivery is made within Ukraine. You can ask questions to consultants by phone. Call us or request feedback.

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Installing piston rings

Remove the set of piston rings from the box. A piston ring set for a Chinese four-stroke scooter usually consists of five items. Three of which are a single assembled oil scraper ring, the other two: upper and lower compression.

A typical set of piston rings looks something like this

Carefully unpack the kit with perfectly clean hands.

In the kit you will find two of the thinnest rings and one thick corrugated ring (expander) - this will be an assembled oil scraper ring that needs to be installed on the piston first.

We place an expander in the lowest groove of the piston

We place thin oil scraper rings in the same groove - one under the expander, the second on top of the expander

We find the lower and upper compression rings in the kit and install them, respectively, in the middle and upper grooves of the piston - with marks on the side surface to the cylinder head (cylinder head). The lower compression ring differs from the upper one in color (not always) and the shape of the working edge (always). In most kits, the lower compression ring is painted black, and its working edge has a slight bevel.

Tools

We free access to the cylinder head: depending on the scooter model, we remove the hood or seat tank. We find the valve cover there, unscrew the four 8mm bolts and remove it from the engine.

The valve cover looks like this. In my case, to get to it I had to remove the hood, saddle and seat tank.

There is a generator casing on the right side of the engine - remove it. If you are too lazy to remove it, just take out the plug. The plug is located in the upper part of the case and can be removed using a regular screwdriver. Through the plug we will control the position of the marks on the rotor.

We insert a 14 mm socket wrench into the center of the generator casing and turn the engine (clockwise) until the “T” mark (from the English “Top” means top, in our case the piston position is at TDC) on the generator rotor is exactly opposite the ledge.

After the mark on the generator coincides with the ledge on the engine crankcase, we check what position the camshaft is in. The camshaft should be in a position where both valves are closed. And it gets into this position every second revolution of the crankshaft since it rotates twice as slow.

The correct position of the camshaft is determined by its sprocket. The sprocket should become the large hole facing up, and the small holes and the marks that are knocked out near them should become parallel to the plane of the valve cover connector.

This is ideal, but in practice the asterisk rarely becomes the way we need it. And the chain is often to blame for this - it stretches and the camshaft begins to slightly not catch up with the crankshaft. If your sprocket has become a little crooked, then by and large it’s nonsense. But if you have it for some reason, then this problem needs to be solved very quickly - otherwise sooner or later the valve will bend.

Once again, check that the “T” mark on the generator rotor is opposite the protrusion, and that the camshaft sprocket is in the correct position. Unscrew the locknuts on the adjusting bolts.

We unscrew the adjusting bolts a little, insert a measuring probe between the valve and the adjusting bolt and, without removing it, tighten the adjusting bolt with our fingers, clamp the adjusting bolt with pliers and secure it with a lock nut.

After tightening the locknut, check how easily the dipstick moves. Ideally, the feeler gauge should move between the valve and the bolt with barely perceptible force.

Photo report: Installing a tuning piston on a Chinese 4T scooter

Installing a piston with a larger volume than stock is perhaps the simplest, most affordable, affordable and effective way to significantly increase the maximum speed and power of a scooter. There are, of course, even more effective ways to increase maximum speed and power, but these methods, or rather the hardware, cost simply unrealistic money...

If you charge the engine of the same 139QMB to the maximum, then such tuning before the “increase” cost the client about 35,000 rubles and this does not include work.

It is clear that not everyone can afford such amounts. Yes, even if it’s affordable, what’s the point? It’s easier to buy an old branded scooter and ride it than to charge the “Chinese” and then throw it in the trash after a season. But that's what I think.

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