Design and technical characteristics of a Minsk motorcycle engine

The MMVZ two-stroke engine with a volume of 125 cm³ has been produced by the Minsk Motorcycle Plant for many years. From 1951 to the present, two-stroke 125 cc engines have been and continue to be produced in various models and modifications. The first motorcycle engine was 4.25 liters. s., at this time, the engine power is 12 hp. With. Structurally, no global changes were made to the internal combustion engine. The cooling remains the same air with two-channel return blowing. Despite the experience of world manufacturers of motorcycle engines using an ejector, the Minsk plant, unfortunately, did not replace the carburetor with an ejector, but it’s a pity, the engine would have had even more advantages.

The Minsk MMVZ-3.11211 has the engine 3.1122-10100-03. In its basic detailing, little has changed structurally in comparison with previous models and modifications of the 125 cm³ engine.

Differences between ICE 3.1122-10100-03 and previous engines:

• Main bearings are installed at the crankshaft cheeks to increase rigidity and improve engine stability, and oil seals are installed behind the bearings. On early models, the oil seals were installed first, followed by the bearings.
• The oil supply to the inner bearings from the crank chamber has been optimized, as well as to the left outer ball bearing from the oil cavity. In older internal combustion engine models, the right bearing was lubricated by oil supply from the gearbox, the left (inner and outer) bearings received oil from the cover cavity. The left inner bearing was not sufficiently lubricated.
• In order to increase the rigidity of the crankshaft, the diameter of the journals was increased from 17 to 20 mm; accordingly, bearings No. 304 were replaced by bearings 6-204 with a higher accuracy class. With a change in the bearing arrangement and an improvement in the lubrication system, the second oil seal on the right side of the crankshaft was removed to protect the generator. The right axle was shortened, at the same time the width of the right part of the crankcase and gearbox cover was reduced by 10 mm. Both parts of the crankcase and the left cover were shortened by 30 mm shorter and their shape was changed.
• To control the oil level in the crankcase, older engines had an oil dipstick; 3.1122-10100-03 does not have an oil dipstick; instead, there is a control hole with a plug.

Technical characteristics of motorcycle engines MMVZ "Minsk" 125

Engine modification

Participation in motorsports requires a reasonable advantage over competitors. Therefore, many will think about how to increase the engine power of a Minsk motorcycle. To do this, you can perform several manipulations:

1. Disassemble the engine.
2. Rings are inserted into the crankcase and securely fastened, reducing the size of the crank chamber by several millimeters.
3. More reliable oil seals with a high load rating are installed.
4. It is recommended to install K-36 (carburetor). It will significantly increase the compression ratio.
5. Ventilation windows are sawn to 20 mm.
6. The length of the exhaust pipe should be increased to 310 mm.
7. Replace the connecting rod in the crankshaft with a duralumin one.
8. Install a larger air filter.

This boost can add more than 7 kW of power to the motor. Overall, the motorcycle is pleasant to move around the city and reliable when doing work. The lightness and controllability of Minsk have long established themselves among motorcyclists. Even now, at a gathering of bikers you can see several modified models of Minsk.

Repair dimensions of the cylinder, piston, rings of the MMVZ Minsk motorcycle engines 125 cm3

Eliminating barking and speeding on a motorcycle

Speed ​​knocking and barking on a motorcycle occurs due to the fact that the gear tooth wears out and the gears begin to repel each other, many blame the gear forks for this, but they have nothing to do with it, the job of the fork is to engage the gear in another gear and then the gears must grab each other , and not push away from each other. This problem can be solved very simply; use a cutting stone or grinder to sharpen the worn teeth at an angle as shown in the photo, and the gearbox of your motorcycle will start working like new.

Photo of how to sharpen a tooth so that the gearbox does not bark and lose speed; the arrow shows a sharpened tooth; this is how you need to sharpen all the gears in the motorcycle gearbox

Repair dimensions of piston pin MMVZ Minsk 125 cm3

DIY motorcycle gearbox repair

Often the problem arises of repairing and restoring a broken tooth in a motorcycle gear, or repairing a gearbox when the speed barks (jumps out and engages and jumps out again) or simply knocks out the speed on the motorcycle. Motorcycles IZH, Voskhod, Minsk, Java, etc. suffer from knocking out and barking speed. Gearbox repair IZH, Voskhod, Minsk, Java, etc. You can do it yourself at home very simply, and restore the old gears of the motorcycle gearbox so that the speed does not bark or knock out.

Weaknesses of Minsk 125 motorcycle engines

• Transmission;
• Kickstarter;
• Carburetor;
• Exhaust pipe mounting;
• Control drive cables (clutch, gas).

The gearbox is one of the few weak points of the engine; problems with speed slippage during the operation of the motorcycle are not uncommon.

The kickstarter (the lever for starting the internal combustion engine with your foot) becomes loose over time at the point where it is attached to the engine starting shaft.

Due to the loss of mechanical properties of rubber parts and wear of the shut-off valve needle, the carburetor loses its tightness and gasoline leaks from it. Of all the carburetors installed at the factory, the Indian P-47R carburetors are considered the most reliable.

The attachment of the exhaust pipe to the cylinder is made in the form of a nut that presses the pipe to the cylinder, which is unscrewed due to vibration.

The cables of the control drives (clutch, gas) must be replaced periodically due to wear or breakage of the cable cable.

Let's start tuning a Minsk motorcycle with our own hands

To increase power we will tune the engine, clutch, ignition system, cooling and exhaust.

Engine tuning in Minsk - adding a few horses

The most powerful engine in the base, as mentioned above, is a maximum of 13 horses. Of course, this is completely insufficient for modern dynamic riding, motocross, and stunt riding (depending on what the motorcycle tuning is for). Modifying the Minsk engine is not very difficult. To increase power, you will have to work on the cylinder and piston of the motorcycle engine. We reduce the combustion chamber by grinding. This increases the compression ratio and engine power. The engine is now suitable for using high-octane gasoline and this will have a positive effect on increasing power.

Motorcycle engine Minsk

So let's get started:

1. It is necessary to change the overall dimensions of the inlet, bypass and exhaust holes in the cylinder to improve the filling of the gas-air mixture and more dynamic release of burnt gases.
2. You can replace your original pistons with Japanese ones with improved performance.
3. A reed valve must be installed at the inlet of the cylinder itself.

Tuning Minsk for sports
Such tuning of the engine of this motorcycle will allow you to add several horsepower to the power , which will immediately noticeably affect the dynamics and acceleration speed. The increased power as a result of engine modification will require replacing the standard crankshaft bearings with more durable ones. For this, it is best to use imported bearings produced by the Japanese company NTM.

Photos of tuning the Minsk 125 motorcycle

Reworking the ignition system

The standard ignition system with a very powerful and reliable generator, a contactless ignition system and an electronic switch require modification in terms of additional installation of a device to change the degree of ignition angle advance depending on the engine speed. With the help of such tuning, the Minsk motorcycle engine will run more smoothly and quietly, and its service life will increase significantly.

Motorcycle tuning Minsk

Exhaust tuning for a Minsk motorcycle

How to make a Minsk motorcycle quieter? To improve the power performance of this engine, it is very important to install a resonator in the exhaust system. A resonator is an oscillatory system where vibration energy is accumulated due to resonance with the driving force. A properly selected resonator allows you to ensure that some of the lost working mixture is returned to the cylinder by the time the exhaust window is closed. It is very difficult to find such a device for free sale, because... it was produced in limited quantities and only for cross-country versions of the Minsk motorcycle. But you can make it yourself using drawings available on the Internet.

Tuning the exhaust system on a motorcycle Minsk Tuning a motorcycle Minsk

Cooling system tuning

Increased power as a result of engine tuning will require a more efficient cooling system. It is necessary to purchase and install a more serious radiator.

Clutch modification in Minsk

Now, after engine tuning, the standard clutch will not be able to cope with the increased power in all operating modes of the motorcycle engine. It needs to be improved by replacing springs and discs. It is necessary to cut off a few millimeters of steel from the standard clutch discs and insert three more selected discs into the free space. As a result of this replacement, the clutch will become slightly stiffer in first gear, but overall the motorcycle will gain more dynamics and drive.

Now we can replace the rear wheel sprocket with a similar one, but with fewer teeth, which will add dynamics to the acceleration of the motorcycle. We replace the standard brakes with good disc brakes - do not forget about driving safety. All these measures, if carried out correctly and responsibly, will allow you to increase the power of a 125 cc motorcycle engine from the standard 13 to 20-23 horsepower, which of course will make your iron horse much faster and more interesting. A technically savvy owner can do the tuning of a Minsk motorcycle engine on his own, or you can contact a motorcycle center, where all the necessary work will be carried out not only quickly and efficiently, but also where you can quickly purchase all the parts and elements necessary for tuning a motorcycle.

Disadvantages of Minsk 125 motorcycle engines

• Vibration during operation;
• Weak power;
• Slow acceleration;
• The need to prepare a fuel mixture of gasoline and oil, which is very inconvenient when traveling long distances. Not every gas station sells oil for MMVZ, so you have to carry it with you.

Conclusion.
Simplicity of design, high reliability, optimal technical characteristics, along with efficiency and safety of operation, long-term maintainability (use of detailing with repair dimensions) made the engine, and with it the Minsk motorcycle, popular among the people. Very convenient for use on the farm, especially in rural areas for going out into nature for hunting, fishing, picking mushrooms and nuts. You can close your eyes to all the listed weaknesses and shortcomings; I say this from my own experience. All of the above breakdowns can be easily fixed with your own hands. There are no problems with the availability of spare parts for sale. It couldn’t be easier to overhaul an engine with your own hands, except that you may need the help of a turner to bore the cylinder to the repair size, followed by honing, if the diameter taper along the cylinder liner mirror exceeds 0.06 - 0.08 mm. Reliability rating is “good”. PS Dear “Minsk drivers”! I look forward to your feedback and comments on the shortcomings and weak points of the 125 cm³ MMVZ engine.

Engine repair

Assembling a Minsk motorcycle engine is the reverse process to disassembly. Therefore, for a clearer idea of ​​the mechanisms, start working with partial disassembly of the motor. Here's a simple procedure:

1. Any repair or replacement of parts must first be accompanied by order. Therefore, before work, rinse the unit thoroughly in water and detergent.
2. Remove the sump cover and remove the clutch mechanism.
3. Remove the cable and return mechanism, disconnect the shaft and remove the support ball.

If the ball is deformed, it is better to replace it with a new one.

4. Remove all contacts coming from the generator, marking in advance where each one was located.
5. Now you need to remove the sprocket with the chain on it. Replace the lock washer with a new one, the shed is probably worn out.
6. Remove the rod from the clutch, then remove the stator by removing it from its mounts.
7. To remove the rotor from the crankshaft, you need to unscrew the fastener 3 turns and hit the bolt several times while holding the rotor. After removal, do not forget to remove the segmental key.
8. Unscrew the cylinder head. The gasket must be replaced.
9. Remove all rings and stoppers from the block. Then remove the finger, placing a wooden block under the cylinder in advance. Be careful, there is a possibility of bending the connecting rod.
10. Now you can inspect the pistons for chips or cracks. Any defects inside the engine must be eliminated.

In this easy way, you can partially disassemble the motor . If there are knocks or jolts while riding, it is worth doing a complete disassembly according to the motorcycle operation and maintenance book. In many cases, even simple cleaning of individual elements helps. For example, cleaning your carburetor will provide better fuel delivery without contaminating the mixture. Replacing the air filter will provide a significant flow of air into the engine, which will improve the purity of the fuel-air mixture.

Advantages and disadvantages

Advantages

• Unpretentiousness . The motorcycle is capable of running on any gasoline, and, most likely, it will even run on AI-76 without any problems, if you suddenly manage to get it somewhere.
• Economical . Spare parts can be found in any village, and they are cheap. And gasoline consumption does not exceed 3-4 liters per hundred kilometers, which is quite good for an ancient two-stroke engine.
• Maintainability . You can repair Minsk 125 Leader on your own, even in a garage, even in an open field, if only you had the tools at hand.

Flaws

• Low reliability . When buying this bike, it is better to prepare in advance for possible breakdowns that you will have to fix yourself. But this is caused more by the age of all surviving specimens than by design flaws.
• Small resource . A two-stroke engine, even with normal maintenance, is inferior in this parameter to a four-stroke engine.
• It is necessary to add oil to the gas tank when refueling . And this, you see, is not very convenient.

Restoring a broken tooth in a motorcycle gear

Of course, it is better to buy a new gear when a tooth is broken, but if the motorcycle is old or imported, it is not easy to find a new gear, but there is a way to restore a broken gear tooth at home. The first time I encountered such a problem with a gear tooth breaking was in my old cross-country motorcycle ChZ-250; two teeth in the first speed gear broke at once. The first time I tried to find such a gear, but I couldn’t find it, and it became very disappointing that because of two broken teeth, my ChZ-250 was at least thrown away or sold for metal. I thought for a long time and decided to try to restore the broken teeth by welding, everything worked out and the ChZ-250 has been running for several years and so far there are no problems with broken teeth. Now I will explain the whole process of restoring broken gear teeth at home without special machines, you will only need welding and a sharpener with a cutting stone or a grinder.

Let's start, in the photo you can see a broken gear tooth on a Minsk motorcycle

Photo. Next, I weld and use electrode No. 3 to replace the broken tooth. I weld in 4-5 passes and after each pass I beat off the welding slag with an electrode, so you should get a metal bulge in place of the broken tooth. After welding, I immediately cool it in water so that there is a slight hardening. But since I’m not a professional welder, it doesn’t look very good, but that’s not important, the main thing is to weld more metal.

Photo. Next, I grind the area on a cutting stone and try to give the appearance of a gear tooth, checking with another gear how the gear fits. You can see the completed work on restoring the gear tooth.

Photo. An important point, since I don’t have a tooth cutting machine and I do everything by eye, of course I can’t grind a perfect gear tooth. Therefore, I grind the tooth slightly thicker than the standard one, and since this tooth is made of softer metal, it rolls as it should while working in the gearbox. The main thing is that after the motorcycle box has been assembled, the gear should, albeit tightly, scroll in the place of the restored tooth, but not jam. A gear with a welded tooth will roll in very quickly.

The gear fork lights up and knocks the speed out of the motorcycle. Video.

Reviews from owners of Minsk Leader 125

A completely indestructible jalopy, it was purchased back in 2003 second-hand for a couple of thousand rubles, I don’t remember exactly, it’s still alive, it’s used for driving around the area in the country. It breaks all the time, can be repaired with a kick or a blow with a sledgehammer, there are spare parts on the market, they are sold almost by weight. The most expensive part that I have changed so far is the carburetor, I installed some Chinese one, I tinkered with it - it works. If only there were spare parts, I think Minsk would even survive the Third World War)) Sergey, Moscow, Minsk Lider 125 '1999.

My first mot, I still remember sometimes. True, mostly with a feeling of relief that I will never see this fiend of hell again. Ironically, now I’m driving a new 250 cc Minsk, heaven and earth in general, and the old one has long been in the trash heap, where it belongs. Ilya, Saratov, Minsk Leader 125 '2003.

A real Soviet legend in a new wrapper, in fact - an ordinary old Minsk with some other little things like a separate lubrication system, a different exhaust and side plastic. If the arms grow from the right place, the motorcycle is practically eternal. You just need to fill it with normal oil, and not M8B, which a normal person would disdain to lubricate door hinges and service it on time. Modern Chinese are no match for old motorcycles, they are all disposable, but the old Minsk is still in perfect order. Anatoly, Ust-Ilimsk, Minsk 125 Lider '2000.