Installation of a 72cc tuning piston on a 139QMB engine

Tuning 139 qmb gearbox

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Tuning the gearbox of a Chinese scooter with a 139QMB engine

Tuning a scooter, in addition to the obligatory increase in dynamics, also provides for an increase in maximum speed.
To achieve the coveted increase of 15-20 kilometers per hour, you need to either rev the engine to high speeds or replace the gearbox. In the first option, in order to ensure the durability of the engine as a whole, it is necessary to stock up on high-quality parts that will easily withstand increased loads. However, this is quite expensive and not advisable, especially on 139QMB engines. A simple way to increase the maximum speed when increasing the piston on scooters with a 139QMB engine can be considered the move to replace the gearbox gears with tuning ones. It is known that a scooter's gearbox reduces the speed of the variator, increases torque and transmits it directly to the rear wheel. The gearbox consists of a set of gears and shafts. Each gear has a certain number of teeth, and this number is calculated by engineers in such a way that a scooter with a certain cubic capacity feels most efficient throughout the entire speed range.

After increasing the piston, the gear ratio of the gearbox is no longer relevant; for the new power it becomes “short”. This means that you will accelerate to the coveted 60 kilometers per hour much more energetically, compared to the stock piston engine, and the scooter will feel much more confident with a passenger, but in this case there is no increase in speed, because the engine speed remains at the same level. Replacing the scooter's gearbox with a tuning one can solve this problem. You can select the most optimal set of gears and replace them yourself, after which the speed will certainly increase, but the dynamics in this case will remain at the stock level.

Each scooter owner must decide for himself what is more important to him, speed or dynamics, and after that start replacing the gears. In some cases, you can achieve a positive effect in both directions, however, after fine-tuning the variator (preferably with a tuning one) and other engine parts as a whole. It all depends on the budget and the tasks assigned to the scooter. Let's consider the main provisions for increasing maximum speed.

How to increase the maximum speed of a Chinese scooter with a 139QMB engine.

The most budget-friendly option for increasing the maximum speed on a Chinese four-stroke scooter with a 139QMB engine can be considered a number of the following changes:

• Replacing the stock 50 cubic centimeter piston with a larger one, usually 82 cubic centimeters with a piston diameter of 50 mm. You can't do without adjusting the carburetor.
• Replacement of old gears of the reducer with an intermediate shaft with new ones. Typically, a stock 139QMB engine is equipped with a secondary pair of gears with a gear ratio of 0.29 and 15x52 tooth gears. By replacing them with tuning ones, the gear ratio of which is 0.35, and the number of teeth of the secondary pair of gears is 17x49, we get an excellent platform for realizing an increase in speed of up to 20%. Modern online stores are already equipped with similar parts and you can easily order them without leaving your home, for example here. The store is proven and has never let you down. Conveniently, they have the gear already pressed onto the intermediate shaft and all you have to do is replace the old parts with new ones. Replacing the original variator with a tuning one. This way you can achieve confident traction throughout the entire rev range and achieve the desired speed. This option can be neglected if after the first two points you have achieved what you wanted. The process of tuning the gearbox on a scooter is quite simple. If you have the skills to replace a CVT, CPG and other parts, replacing the gears will be an easy task. The design of the 139QMB engine allows access to the gearbox only after removing the rear wheel, removing the variator cover and clutch. Before doing this, you need to drain the old transmission oil. You need to drain the oil while it is “hot”, preferably after a trip so that the gearbox is hot. Sometimes you will need an impact driver to remove the screws.
• Replacing gears on a 139QMB engine

All operations must be carried out carefully so as not to damage the gasket. After replacing parts, the screws must be tightened alternately in several passes in order to distribute the tightening force on the gasket as much as possible. It would also be useful to use an impact screwdriver here.

After assembling the gearbox, fill in transmission oil and listen to its operation. There should be no extraneous noise or crackling, and the wheel should not jam. After replacement, the gearbox also requires a kind of running-in so that all the parts get used in order to form the correct gap between them necessary for operation. Do not rush to anneal it at maximum speed right away; let the unit operate for about 100 km without loads or overheating.

DOCUMENT containing useful information on the 139QMB engine:

Source

3 Features of improving the carburetor, cylinder head and variator

After disconnecting the plugs, you need to proceed to the next tuning steps. To begin with, it is recommended to replace the factory muffler with an improved version with a resonator. Well-known models include Laser X, Technical Next or Leovince. After installing the new pipe, the transmission must be properly adjusted. Such changes will increase the maximum speed by 7-10 kilometers per hour and improve acceleration dynamics.

Next, you should install a new carburetor with an increased diffuser diameter. For models of Chinese mopeds with a cubic capacity of up to 150 cm3, it is best to purchase carburetors with a stroke diameter of 17.5-20.5 millimeters. Which manufacturer to choose is up to the owner to decide, but some experts advise purchasing models of inexpensive and fairly reliable carburetors produced by DellOrto. When carrying out all of the above work, it is also recommended to install a tuned variator; it will be easier to adjust it yourself to new parameters, since it has a more advanced structure. You can choose models from the Italian manufacturer Polini, which are equipped with a special lubrication system for the main rod.

But the most effective way to really increase the power of a small-capacity scooter is to install a new cylinder-piston group of increased volume. It is important to remember that if you are tuning a scooter with the installation of an improved piston group, it is strongly recommended to check the degree of wear of the crankshaft and bearings, since if they are in poor condition, tuning will completely “kill” them after 1000-2000 kilometers with new parameters. This is where many people who want to tune their scooter stop, especially when it comes to an engine like 139qmb. Further actions can be attributed to racing tuning, and in terms of costs and volume of work they are comparable to purchasing a new, more powerful and voluminous scooter with 150 or 225 cc with sports characteristics.

Tuning the gearbox of a Chinese scooter with a 139QMB engine

A simple way to increase the maximum speed when increasing the piston on scooters with a 139QMB engine can be considered the move to replace the gearbox gears with tuning ones.

It is known that a scooter's gearbox reduces the speed of the variator, increases torque and transmits it directly to the rear wheel. The gearbox consists of a set of gears and shafts. Each gear has a certain number of teeth, and this number is calculated by engineers in such a way that a scooter with a certain cubic capacity feels most efficient throughout the entire speed range.

Why install a tuning gearbox on a scooter?

Most scooter owners complain about the lack of top speed after tuning their scooter. That’s right, you improve the engine’s power performance, install a tuning switch that no longer cuts off engine speed, but the maximum you get from this is an increase in speed of 20-25 km and excellent dynamics.

In essence, you are tuning your scooter for dynamics in this way. Now it’s easier for him to accelerate, drag two people, and climb a mountain.

Preliminary work

Before repair work, the most initial, necessary diagnostics of engine components, inspection of repair accessories, assessment of weather conditions, etc. are carried out. To summarize, the list of preliminary work is as follows:

• The motor of the bike must be cold; it is advisable not to ride it before repairing;
• It is necessary to clean the engine elements from soot and dirt in advance, since dirt can get inside the valves;
• The temperature in the garage (or other place where repair work is carried out) must be above zero;
• The bike is rigidly fixed on the stand or in other convenient ways;
• The necessary set of keys and containers are prepared and are at hand.

Analogues - engine 1P39QMB

Chinese developers have created an analogue of the Japanese 139QMB engine - a motor labeled 1P39QMB, which in appearance completely replicates the original. Despite all the similarities, you can still find differences: the valve clearances on the 1P39QMB are not adjustable. The situation is similar with the carburetor: before direct operation it requires thorough cleaning and correct adjustment. Chinese copies of 139QMB engines, of course, cope with their task, but their main purpose was to reduce the final cost of motor vehicles. Budget versions of scooters are equipped with exactly the same versions of engines that are good only for short trips at low speeds.

Before starting operation, be sure to carry out a full run-in of the 1P39QMB engine. The optimal operating mode of the engine begins only after 2 thousand kilometers, but after 10 thousand kilometers all its technical characteristics drop, and it loses dynamics and power.

We adjust valves (4t engines).

As mentioned earlier, the engine must be serviced before boosting. Work that will help improve dynamics includes adjusting the valves. The procedure is suitable for tuning Chinese 4t scooters and other 4-stroke engines. For small-capacity equipment, it is recommended to carry out such adjustments at least once every 3, and preferably once every 2 thousand km. It is also necessary to carry out adjustments after manipulations with the CPG.

Before you begin, you need to make sure that the engine has completely cooled down and that you have a set of keys (8,9) and feelers of 0.05 for the intake valve and 0.07 mm for the exhaust valve at hand. You can get to the adjusting nuts by first removing the valve cover. When access to the valves opens, the piston is installed at top dead center. Next, a feeler gauge is installed in the intake valve gap and, depending on the current position, an adjustment is made.

To increase or decrease the clearance, you need to loosen the lock nut (while holding the adjusting bolt), turn the adjustment bolt and, while holding it, tighten the lock nut back. Thus, we achieve a gap when the probe passes into it with little effort. After adjustment, make a full revolution of the crankshaft and make sure that the adjustment is accurate.

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.

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