Do-it-yourself Ural chainsaw engine repair

In detail: Do-it-yourself repair of a chainsaw engine from a real master for the site my.housecope.com.

I bought a chainsaw Ural 2 Electron. The instruction manual for this instrument has been lost. Can anyone describe the procedure for adjusting idle speed, starting throttle position?

In my tool, I adjust the idle speed by changing the length of the damper cable. Those. coarse adjustment I unscrew the screw holding the cable and pull or loosen it with my hands; for fine adjustment there is a stop screw near the throttle grip.

I am repairing the Ural 2T Electron chainsaw (1988 onwards). I bought a magneto, put it on - do not start, sneezes and tugs the hand so well (gives it to the side opposite to the cranking motion by the starter). Well, I think the ignition timing needs to be adjusted. Results - he pulls the hand earlier, then later. I have a question. How does she start? Or am I doing something wrong?

In my practice, there was once a similar case. Only when the engine kicked back - cotton and smoke fell from all sides (both from the carburetor and from under the crankcase too). Although it happened and got started. It turned out that the pin on the piston, which keeps the ring from turning, has weakened, and since it is located opposite the bypass window, it has hollowed out a groove in this window with a length of about 6-8 mm. Yes, plus an underworked piston, as a result, the bypass window opened before the exhaust. Examine the inside of the cylinder, maybe that's the case. Insert a thick sheet of paper there (along the height of the cylinder) and circle all the windows with a piece of pencil - you will get a scan of the cylinder, then compare it with a piston. That's how I defined this problem then. And also look: the piston in the connecting rod does not play much? How do the rings feel there (especially the top one)? And try the compression just in case.

Video (click to play).

It is not necessary to move the magneto further than the grooves, all the more to make a new groove in the flywheel. Most likely, the magneto is buggy: either at the factory there is something with the control coil contrived (the wrong side, not the number of turns, the input-output is confused), or the thyristor burned out and opens ahead of time. In any case, it is advisable to check the magneto on a working chainsaw and decide whether it is magneto or not. You can also try to turn the wires of the control coil or itself, or by placing spacers under the magneto, try to move it along the flywheel axis (maybe the control magnets are overlapped there somehow by the power ones) Although this is just my thoughts. And also as an option: try to restore the old magneto.

Why does the Ural Electron chainsaw engine not provide the required power?

Perhaps the piston rings are burnt in the piston grooves or the ring is broken. There may also be leaks in the joints of the cylinder to the crankcase, the carburetor to the cylinder, wear and loss of elasticity of the rubber crankcase seals. And another reason - the holes of the carburetor nozzles are clogged.

Is the chain sharpened on the machine, or can it be corrected only by hand?

The Oregon chain for Ural-2T ("Druzhba") has the name 0.404 ″ 64 1.6mm 20 ″ / 50 (Ural, Druzhba) longitudinal and model ORG / 27R64E.

I have this tool in operation. There was such a problem: when the fuel tap is open, gasoline flows from the carburetor, I think, because of this, I can not adjust the carburetor. Maybe someone faced such a problem? Gasoline flows into the diffuser between the air valve and the throttle valve. The hose was changed. Started after washing the carburetor.

Disassemble it again, buy a repair kit, replace everything except the valve, it is not in the repair kit. There may be small chips in the orifice where the valve is inserted. Maybe because of it, the gasoline is leaking.

I am adjusting the carburetor on the Ural 2 Electron chainsaw. Unscrew the idle screw by more than 2.5 turns. This is fine? A situation arose: I adjusted it while everything was fine on the ground, as soon as I lift it, it starts to gain momentum, I will lower it to the ground again normally. Then he twisted it again, began to work normally. Most likely, the oil seals will have to be replaced. And how long do they serve on average?

This unpleasant situation arose from the fact that the clamp does not firmly grip the gearbox with the crankcase, and when you lift the saw, the tire and the gearbox move the crankshaft with their weight, resulting in air leakage. Is it necessary either to put rubber at least 5 mm in, or to buy a new clamp, just before these two options, check whether the clamp fastening bolt is fully clamped? If the screw is not tightened to the stop, then first release the clamp of the clamp, tighten the screw, then tighten the lock back. This is the problem.

Why on the Ural 2T chainsaw, the driven sprocket on the tire rotates tightly, with jamming?

Most likely, dirt, sawdust or tar got into the sprocket bearing. It is necessary to thoroughly rinse in gasoline or kerosene, lubricate with oil.

There was such a problem. I put a repair kit for the carburetor b / p Ural, replaced everything. Now, with great difficulty, it starts up and you gasp twice and stalls. What can be done?

Did you adjust the carburetor after installing new parts? - stalls with a ringing sound or deaf? After replacing the repair. kit, be sure to adjust the carburetor, "from scratch", when assembling the carburetor with a new rem. kit, the jet was replaced (if a complete repair kit), if so, then it was necessary to adjust the valve arm that closes and opens the jet (there is a spring under this valve). In theory, if the new jet, it now does not open normally during robots, in addition, check that this new jet is not clogged, or rather, you could get defective rubber in this jet, which softened a little from the fuel and closed the hole.

Saw Ural 2 electronic ignition. I stood for two or three years without work, yesterday I wanted to start it up, but with great difficulty worked for a few seconds, the same story died out when I started it again. Maybe someone will tell you what is wrong?

Maybe before putting it into the cylinder, oil was poured. Try to unscrew the candle, wipe clean it from carbon deposits. With the spark plug removed, the air and throttle valves open, turn the crankshaft with the starter several times. Maybe the carburetor was misaligned in the past, try adjusting it according to the instructions. when did you start smoking heavily or not? Old gasoline can be affected, so it is better to replace it, do not forget with oil.

Because of what the Ural 2T Electron chainsaw can fill in candles? The spark seems to be good.

It can be poured if: the gap in the candle is large (for a contact magneto it should be 0.5, for BSZ - 0.6-0.8); more oils than put in the mixture; cold candle (A17 instead of A11); the spark is weak. About the carburetor - even if it is incorrectly configured, it should start (at least KMP-100). This is if it is serviceable.

On my chainsaw, the saw chain does not stop idling. What could be the reason?

Either the idle speed has not been adjusted, or the clutch has not been adjusted.

I operate the Ural chainsaw. I poured fresh gasoline under the pumping with the air damper closed, turned the crankshaft with the starter, then opened the damper, it started up and while trying to adjust the carburetor it stalled for me four times and for the fifth time it stalled completely. Then I tried to start it and nothing happened with the shutter open and closed. After all this, I purely accidentally tilted the saw towards the muffler and gasoline poured out of there. What could be?

This is really weird. First of all, ventilate the engine, close the fuel valve, open the air and throttle valves, unscrew the spark plug, and turn the crankshaft several times, while it is imperative that a spark plug is installed on the high-voltage cable, which in turn is reliably connected to ground. How much to rotate the crankshaft? it's hard to say, since the overflow is huge, first, after 15-20 jerks, probably “drain” by tilting the saw off the muffler, when the fuel stops flowing out, move away from the place where it was drained, and after the same 15-20 jerks, burn the fuel in the muffler.If after the next portion (15-20 jerks) the fuel does not ignite in the muffler, you can try to start it again. The reason for such an overflow, I can assume, if the saw did not start for a long time, the oil could settle on the valve of the overlapping nozzle or on the nozzle itself, as a result there is no tight closure, in addition, due to a long idle time, the membrane could dry out in places, and after starting the engine it could crack , which could also lead to a leaky closure of the nozzle. I recommend checking the carburetor.

Oil leaks from the oil tank on the Ural 2 chainsaw. How can I fix it?

Put on one normal store gasket (like paronite), you can try a sealant. Regarding the increase-decrease in revs: I also toiled with this. Replaced the front oil seal and became normal. Try to remove the gearbox, you will immediately see if the oil seal passes everything black and wet there. Although if you removed the ignition, then changing both front and rear oil seals will not be superfluous for sure. The gasket between the carburetor and the cylinder may be worn out.

The tool cuts very hard. What could be the reasons and how to fix it?

There may be several reasons: The teeth of the chain are dull - you need to sharpen it. No lubrication to the chain - Check the presence of oil in the oil tank, clean the channels of the lubrication system. The chain is too tight. The chain tension needs to be adjusted. The bar may be bent or the chain is dirty. Wear of the tire blade along the working part. You need to flip the tire.

Why does a chainsaw cut obliquely?

Most likely, the cutting teeth on one side of the chain are higher or lower than the other side. Sharpen the chain correctly or replace it with a new one.

Why doesn't the chain stop idling?

There are several reasons: Idle speed is not adjusted, the clutch is not adjusted.

Chainsaws "Ural" have been known for more than half a century: their production began in 1955 at the Perm plant, which had worked for the country's military-industrial complex for many years. Until now, their popularity has not faded: demand still remains high despite the abundance of new foreign brands. Some do not want to overpay: the purchase of the Ural chainsaw is more budgetary than similar devices from leading European manufacturers. The same applies to their maintenance, repairs, which save the family budget.

But most of the users of these chainsaws appreciate the traditional quality, which has been tested over decades of service by more than one generation of people.

We will get acquainted with the features of the legendary saws of this manufacturer, their structure and functionality, as well as with the nuances of maintenance and repair, in this article.

Chainsaws "Ural" - the successors of another famous brand "Druzhba", which are considered its improved counterparts. Compared to Druzhba chainsaws, they have:

high power;
gear equipment;
removable container for chain lubrication - crankcase;
improved cylinder and starter;
easily removable cutting part of the saw;
upper handle composition;
Hydraulic wedge has been added to the construct.

While there are many advantages to this technique, it is not without its drawbacks. The main ones are:

a large amount of toxic exhaust;
frequent clogging of the air filter;
high gas mileage;
from here come frequent problems with the engine and gear part

Consider the range of gasoline saws from this manufacturer.

Gasoline hand tools designed specifically for tough working conditions and tasks of increased complexity.

The device of the Ural 2t Electron chainsaw is quite simple. The main working details are:

engine;
starter;
reducer;
switch;
cutting headset;
gas tank.

There is also a steering wheel and an element on which the structure can be supported.

Technical features of the chainsaw "Ural" 2t Electron

2-stroke engine that consumes 632 ml of fuel per hour of operation;
high power - 3.68 kW;
46 cm tire with detachable chain;
heavy weight - 11.7 kg.

A productive model with improved functionality. The inertial chain brake and chain stroke in auto mode simplify the operator's tool maintenance tasks.

propulsion thrust - 3.9 kW;
capacious gas tank - 0.55 l;
volumetric oil sump - 265 ml;
maximum bus length - 0.45 m.

Sufficiently compact and lightweight chain chainsaw. Decent semi-professional unit, adapted for medium loads.

It differs from other modifications:

thrust of 3.8 kW;
large tanks for fuel and chain oil - 0.62 and 0.25 liters;
standard chain pitch - 3/8;
long bus - 35-45 cm;
light weight - 6.89 kg.

Professional-grade petrol power tool. Features improved grip, chain brake and power button lock function.

Users note that this model of the Ural chainsaw breaks down less often than others and requires repair.

Industrial grade chain saw. Forces of 5.1 hp enough for all woodworking.

Gasoline tank - 0.55 l;
Carter - 0.25 L;
Tire - 45 cm;
Weight - 7.8 kg.

It is advisable to begin acquaintance with such a technique with the instruction manual. Covers such issues:

the internal structure of chainsaws;
the nuances of refueling;
running-in and rules for starting the engine;
preparation of the device for sawing;
care and storage;
safety standards.

Let us dwell on some of them that can be encountered in the process of using Ural chainsaws.

Resourceful people have long figured out how to convert their outdated Ural models to 92 gasoline. To do this, you need to lower the octane number of AI-92 or AI-95 gasoline by adding 70 g of diesel fuel to one tank of the fuel mixture. The engine practically does not feel the difference with such an additive and operates normally.

The proportion for the gas-oil mixture should be as follows: 50 ml of engine oil (preferably not for mopeds) per 1 liter of gasoline.

A serviceable and filled instrument can be started. For this you need:

put the switch in the working position;
pump the fuel mixture into the carburetor;
pull the starter;
warm up the engine and get to work.

If the device is completely new or returned from repair, it must be run in, which we will talk about further.

the proportion for the break-in mixture is 15: 1, i.e. the amount of lubricant increases;

the first 25 hours of running-in should start with idle speed, which is gradually replaced by minimum and average working revolutions;

this process ends after 4 full fuel mixture tanks have been consumed.

For successful operation, it is necessary to adjust the tool - adjust the idle speed and the ignition system so that it gives out the power put into it, works economically and breaks less often.Before leaving the factory, all chainsaws receive factory settings, which are retained for a certain time. After purchasing new equipment, the question of how to adjust the carburetor on the Ural chainsaw becomes central. Do not rush to readjust the carburetor after purchase: let the saw run for several hours at low revs. Gradually adjusting the jet clockwise, they switch to high revs, during which the saw should not hum and smoke much. If this happens, the screw is tightened: the fuel mixture will become leaner. Following this principle of adjustment, the saw can always be adapted to any load.

To do this, unscrew the nuts securing the cylinder to the body. Carefully remove the cylinder from the piston rings and carefully inspect the piston, rings and cylinder.

Photo-2 Carbon deposits on the piston and piston grooves.

Traces of heavy carbon deposits are visible on the piston and piston grooves, which must be removed, and carbon deposits are also present on the inside of the cylinder.

Photo-3. Removing the piston rings from the piston.

To remove carbon deposits from the piston grooves, the piston rings must be removed from the piston. The rings and piston are usually soaked in kerosene or gasoline until the carbon is softened. Broken or badly worn rings should be replaced with new ones.

Photo-4. Removing carbon deposits from the piston.

A wooden stick or rag can be used to remove carbon deposits from the piston, but not metal objects.

Photo-5. Removal of carbon deposits from the piston grooves.

To remove carbon deposits from the piston grooves, you should also use non-metallic objects. After removing carbon deposits or replacing the piston rings, reassemble the chainsaw in the reverse order.

Another reason for a chainsaw malfunction may be severe wear of the ratchet, which is installed on the flywheel bushing of the chainsaw crankshaft. As a result of such a malfunction, it is not possible to start the chainsaw engine, since the starter ratchet does not cling to the chainsaw ratchet. To do this, disassemble the deflector, remove the crankcase cover, then unscrew the ratchet and replace with a new one.

To remove the deflector, it is necessary to unscrew all its fastening nuts. But first, you must remove the frame with the gas tank, disconnect the gearbox from the engine, and also disconnect the gas line from the carburetor.

Photo-7. Removing the crankcase cover.

In order to remove the crankcase cover, it is necessary to unscrew the fastening bolts, then pull the cover carefully towards you.
Photo-8. Unscrewing the ratchet.

To remove the ratchet, it is necessary to block the rotation of the engine crankshaft. This can be achieved by locking the friction clutch on the crankshaft from the back (from the gearbox side).

The ratchet can be unscrewed easily, for this use a wrench. All parts (deflector, crankcase cover) are assembled in the reverse order. That is, first put the crankcase cover in place and secure, then install and secure the deflector.

If, while cutting a chainsaw, the gearbox makes a grinding or other extraneous sound, then it is necessary to disassemble and inspect it. It is possible that one of the two bevel gears of the gearbox is out of order. The gearbox should be disassembled from the side of the drive sprocket of the saw chain.

Photo-10. Removing the leading sprocket.
For more detailed information on the rules of operation and methods of disassembling the chainsaw, download the instruction.

In order to remove the driving sprocket of the saw chain, unscrew the fastening nut and pull the sprocket towards you. The sprocket should be carefully inspected, in case of severe wear, replace with a new one.

Photo-11.Dismantling the driven bevel gear.

To dismantle the driven bevel gear of the reducer, it is necessary to unscrew the fastening nuts and pull the gear housing towards you.

Photo-12. Replacing the driven bevel gear.

The driven bevel gear of the reducer is inserted into an aluminum housing with a bearing. If necessary, disassemble the entire structure and replace worn parts or replace the entire structure.

In the event of increased noise during the operation of the chainsaw, this may mean that the muffler has been destroyed. In such cases, the muffler should be replaced.

To replace the muffler of the chainsaw, unscrew the two M6 fastening nuts with a wrench, then unscrew the two M5 screws.

Photo-14. Unscrew the M 5 screw for the muffler.

Use a screwdriver to unscrew the two M 6 screws.

During the operation of the tool, it is very likely that the following problem may occur:The engine starts with difficulty or normally, but it does not work stably, intermittently. There are several probable reasons in such situations, we list the most basic ones a little below:1. Poor quality fuel or water got into the fuel.2. Due to low-quality fuel, the carburetor clogged, from frequent attempts to start the engine, the spark plug was moistened with fuel and a lot of fuel accumulated in the crankcase.3. Leakage in the joints between the cylinder and the crankcase, and leaks between the carburetor and the cylinder may also occur.4. Rarely, but still happens-depressurization of the crankcase of the chainsaw.The methods for eliminating such causes are as follows:If the reason is low-quality fuel, then all residues of poor-quality fuel should be drained from the gas tank.disassemble carburetor, clean the carburetor mesh filter from debris and reassemble it, reinstall it. Adjust the carburetor as follows:1. Screw in the full throttle screw of the carburetor as far as it will go and then turn it back 1/2 turn.2. Tighten the idle screw as far as it will go, then back out one turn.The leakage of the connection between the cylinder and the crankcase or the carburetor with the body of the chainsaw is eliminated by tightening the fastening nuts. Unscrew the spark plug and dry and clean. Gasoline from the crankcase must be blown out as follows:Close the fuel valve, then unscrew the spark plug, install the starter and pull out the starter cable, then release it back. This operation is repeated many times until all the fuel accumulated in the crankcase is blown out (fuel is ejected through the spark plug hole on the cylinder).Photo-15. Disassembly of the driving clutch. Image - Do-it-yourself Ural chainsaw engine repair

To restore the tightness of the crankcase of the chainsaw, it is necessary to disassemble it. To do this, the first step is to disassemble the drive clutch.

Photo-16. Removing the bearing cover.

In order to remove the bearing cover, it is necessary to unscrew the fastening nuts without damaging the gasket. Then remove the adjusting ring that is installed between the bearing and the cover.

Photo-17. Dismantling the engine crankcase.

We continue dismantling, it is necessary to unscrew five nuts from the studs and disconnect the left side of the crankcase from the right side.

In this case, as a rule, the gasket always fails, since it is paper. Next, you should draw another gasket on drawing paper, then lubricate it with oil and install it back on the studs.

Photo-19.Installation of a new gasket on the studs.

We install a new gasket made of drawing paper on the crankcase mounting studs, then take it out and grease it with oil.

Photo-20.Connection of the left side of the crankcase to the right.

After that, we install it back on the studs, then we install the second part of the crankcase and fasten it with fastening nuts. The disassembly of the engine crankcase is performed not only when it is necessary to replace the gasket, but also when the worn out bearings, as well as the connecting rod and piston group are replaced. chainsaw Ural

More recently, I had to deal with repairohm engine a well-deserved hard worker - chainsaws "URAL 2T Electron".In this article, I want to outline not only the operations of disassembling and assembling a particular engine, but I will try to convey in the most accessible form the general methods, techniques and little tricks for repairing two-stroke engines, using photographs as illustrations.Enginecaught on repair, started only on a highly enriched mixture for a short period of time and stalls. The compression was good. Adjusting the carburetor and ignition timing had no effect. It was suggested that the malfunction was caused by air leaks before the compression stroke. The gaskets under the carburetor, cylinder and in the crankcase looked intact; crankshaft oil seals were suspected.But what needs to be done before proceeding with the disassembly of a unit or unit?-first, you need to clean it from dirt and wash it,-prepare the workplace and the necessary tools,- during work, observe caution and the required minimum of safety rules. Take care of yourself.The saw motor can be disassembled quite simply, the only difficulty is removing the flywheel, for which, according to the instructions, you need to use a device. Instead of this device, I cost 3 10 kopeck coins, put them under the ratchet, on the end of the crankshaft and, having tightened the ratchet well, struck through the hole with a hammer through a blunt guide on the coins. One hit was enough.So, engine disassembled.A little advice. When disassembling, if possible, fasteners (nuts, washers, etc.) try to bait in their places (you can see this in the photo), and what does not work, put them in different jars, boxes and remove them away from the place of work.In order not to turn it over, not to be confused and not to be lost. By the way, the savings in time and nerves are enormous.My guess about the cause of the malfunction was justified.The spring on the front crankshaft seal has become disconnected.We clean the cylinder channels and the piston bottom from carbon deposits (do not touch the side walls and the piston skirt). If the replacement of the compression rings is not planned, then we leave them in their places, where they were worn in.We check the free movement of the rings in the piston grooves, the condition of the crankshaft bearings. After that, you can start assembling.We install oil seals, gaskets. Apply the sealant in a thin layer (the principle "you can't spoil the porridge with oil" is categorically inappropriate here).Having lubricated the edges of the oil seals with engine oil, we install the crankshaft, not forgetting to reinstall the spring ring.We give the sealant (according to the instructions) an exposure in air for 15 minutes and connect and tighten the halves of the crankcase.The end face of the gasket protruding onto the cylinder seating plane is carefully cut flush with a sharp knife.Install the cylinder gasket by orienting the gasket cutout towards the carburetor to align with the cylinder bore.We pass a high-voltage wire into the hole in the crankcase, screw it into the hole of the high-voltage transformer. We fix it securely so that it does not fall out spontaneously.Then, pulling the wire back, we put the magneto in place, so that the risks on the crankcase and on the magneto coincide.and firmly fasten with special nuts, with a cut for a screwdriver. We install the flywheel, not forgetting to put adjusting rings under the ratchet, the size from the end of the ratchet teeth to the end of the crankcase cover is 21-22 mm. Image - Do-it-yourself Ural chainsaw engine repair

Chainsaw engine "URAL 2T Electron" assembled.

Ural Electron 2T Operation manual
Hello everyone! By the will of fate, he became the owner of the Ural Electron gas-powered saw. Thanks to the five same will of fate, the operating manual of this tool was lost. Gentlemen, does anyone have an electronic version of this document, or, at worst, can someone describe the procedure for adjusting the idle speed, the initial position of the throttle valve?

[RIGHT] ————-
Operating instructions are in the posts and [/ RIGHT]

In my "Ural" I regulate the speed of h.h. by changing the length of the damper cable. Those. coarse adjustment I unscrew the screw holding the cable and pull or loosen it with my hands; for fine adjustment there is a stop screw near the throttle grip.

Dmitry, thank you, I understood you. But it seems to me that I still have to turn two screws to achieve the desired quality of the air-gasoline mixture: the spark plug electrodes are blackening, which indicates a rich mixture. How to proceed here?

Quadruple I had "Friendship electron". From memory, I just twisted the jets all the way, and then twisted them 1.5 turns. He started and already twisted XX (if necessary). I think that in the Urals as well. Well, if you want to know more precisely, contact any major service center and they will explain.

The four-breeder wrote:
... the spark plug electrodes are blackening - which indicates a rich mixture.

For a two-stroke, this is normal - a mixture of gasoline and oil 🙂

Father-in-law gave a Ural 2T chainsaw

Advise how to be. Father-in-law gave (gave) a saw Ural 2T Electron (1987 century) with the words drank sons-in-law for health, buy only a magneto and work. Brought the saw for 400 km. from the village (I want to cut firewood in the garden). I bought a magneto, put it on - do not start, sneezes and tugs at the hand so well (gives it to the side opposite to the cranking motion with the starter). Well, I think the ignition timing needs to be adjusted. Results - he pulls the hand earlier, then later. I have a question. How does it start? Or I'm doing something wrong.

2Hmel Read first here
” >

Was reading. I installed this ignition according to the book that my father-in-law gave - no result. The whole problem is contactless ignition. On a motorcycle, everything is simple - brought it to top dead center, lowered it by 3 mm, made a break in the breaker, everything works.And in contactless ignition, how to find out at what moment a spark occurs.

2Hmel There should be a mark (dash) on the magneto, on the motor case too.

Here's what they have written:
“At the factory, when setting the ignition timing against risk 7 (Fig. 4), risk 7 is placed on the magneto on the crankcase (Fig. 4). When installing the magneto after replacing it, after routine or repair work during the operation of the chainsaw, combine the indicated risks. "

IMXO if nothing else is violated, then this will lead to an advance = 3.9 mm.

You can reduce this lead to, say, 3-2 mm by turning the magneto in the grooves, but by doing so we lose power.

And the recoil can be due to many reasons.

Can you weakly (slowly) pull the starter rope?
Or maybe there is soot on your finger on the piston?
Or maybe the keyway is broken?
And the candle in what condition (gap, changed, did not change), gasoline and so on and so forth.
It may even be that this magneto is (in our time, what they want to vparyat).

In general, there is too little information for more specific advice.

2Hmel In my practice there was once a similar case. Only when the engine gave a return - cotton and smoke fells from all sides (both from the carburetor and from under the crankcase too), although it used to start. It turned out that the pin on the piston, which keeps the ring from turning, has loosened, and since it is opposite the bypass window, it has hollowed out a groove in this window with a length of about 6-8 mm. Yes, plus an underworked piston, as a result, the bypass window opened before the exhaust. I suffered - horror, but I won.

Examine the cylinder from the inside, maybe the dog rummaged there? Put a thick sheet of paper in there (along the height of the cylinder) and circle all the windows with a piece of pencil - you will get a scan of the cylinder, then compare it with a piston. That's how I defined this byaka then. There will be difficulties - take a picture of the scan and put it here, maybe I can advise something else.

And also look: the piston in the connecting rod does not play much? How do the rings feel there (especially the top one)?
And try the compression just in case.

As a version: you have a flash very early, because of this, it throws the piston immediately back and, accordingly, jerks the hand. - Draw conclusions: either the magneto is not right, or you put it wrong - the angle is not the right one. The flash should not be very early, otherwise the piston will not pass the dead center.

Thank you, there are two days off ahead, I'll watch. Research results on Monday

Video (click to play).

Maybe all the same there are magnets on the flywheel. I haven't come across them myself, but I heard them. but what if?
And the link mentioned them.
By the way, I have never heard that boring grooves on the ignition block saved someone