Egor....HELP

I did the mods you suggested: trimmed the piston skirt (per your pic), I raised the exhaust 1.1" from the top, and lowered the intake .25", I did the stock carb mod you showed, and I changed to the bottom notch on the needle to allow max fuel flow. I got her started and immediately had to set the idle all the way in to keep running. Once warmed up, I went for a ride and now I have much less bottom end (will barely get up to 20 mph on flat land) and almost no torque at any throttle range. Should I have left the needle where it was? Any suggestions would help....J

I would try the needle in the stock position, cant hurt.....

Look for air leaks
Peace

Grinding the piston skirt is one way to alter the timing. Grinding out the bottom of the intake port is another way to do the same thing. I wouldn't advise doing both though. I don't think this is your problem however.

I would admonish anyone thinking about doing these modifications to study and understand why this increases hp. Understand how the 2 cycle motor works and how altering the ports changes the characteristics. It is very easy to go too far and mess up a good engine. Don't even start to make these changes unless you understand the why of it. Port shape is also important as it is possible for the rings to get stuck on a poorly altered port.

If you're going to take the cylinder off the bottom end it is also good to know how to put it back together with the rings aligned with their respective keeper pins.

Lastly I would suggest performing any changes in steps as it makes troubleshooting much more difficult if you do it all at once. If you had made the carb modifications first and then tested the bike you would now know if that is what is causing your problem or not.

For now I would start with making adjustments to the needle and testing.

I have done these same things to dirtbikes over the years...seems to have lost compression big time, air leak may be it, but I RTV'd the bottom end...what's the compression supposed to be on these motors? I can put a tester on it and see if I've lost comp., or its a carb problem.

J - I think you have increased the intake timing too much. The two ways to increase it is to cut the piston skirt, (or) lower the intake port. The best is to lower the port you get an increase in flow that way. I am sorry for the missed information, you will have to get another piston. Or, If you want you can turn the piston around there is no front or back to this engine. I have gotten two engines from the manufacture with them ether way. On some high end engines the pin is off center and this can not be done, but on the happy time that is not the case. You will notice that the rings will not get into the ports also in ether position they stay in the clear ether way. Let me know how it goes, and what you decide. Sorry again for the missed information. Have fun, Dave

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Chuck - The monkey has the spark plug again!

thanks for the advice...a couple more q's: is RTV by itself enough to seal these cylinders? my gasket was in tatters when i removed it, so the RTV was all I did. How does increasing the timing too much reduce power? Shouldn't that have given the engine too much low end rpms? Not doubting you, it just feels like a leak, what with the lost compression and all. I will flip the piston and see how it goes. thanks, J

I did use just RTV but I let it cure for the day, and I just used enough to make a nice small bead. As for the timing, on a piston ported engine, as the piston goes up it is in-taking. This is the crutch as the piston is going down it needs to compress the air it just took in, if the port is open too long it just pushes it back out the carb, you have to find the middle of the road. The set up you have with the longer time would be best for a reed induction intake. If you could get one of those going you would like it, there is some blog around about that. Make sure you are lining up the rings with the two little pins in the piston. Have fun, Dave

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Chuck - The monkey has the spark plug again!

Well, I flipped the piston around, and set the needle back to the stock position. I have power again, but still not the same low end torque as it had before I did anything, which is a bummer because that's the whole reason I did the mods. I'm running a 34T sprocket, and while it wasn't a big hill climber before, at least it wouldn't bog out with a little pedal assist. Now, any incline seems to slow me way down. I don't notice any air leaks, and now I'm wondering if it was the "stock carb mod". I dunno...I had visions of motoring up hills with the modded motor, I should have just left it stock.

J - You should have increase of torque and HP. It should idle like an expensive motorcycle, I can climb a hill out here that you would not believe, and with a 39 tooth sprocket. How did the parts look when you turned the piston around? Was the engine running good when you started the mods? You might need to look for a gasket leak that is the biggest killer. The carb mod has never failed adds fuel to the idle position that's all, will not affect the rest of the carb settings. Is the engine fussy at idle IE very hard to keep running, that's the sign of the air leaks. keep us posted. Have fun, Dave

PS: With the engine running spray WD40 around the suspected places that might leak air, if the engine starts to speed up you have found the leak. There are the crank seals that fail also, I have had two go bad. I have done these mods to all five of my engines.

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Chuck - The monkey has the spark plug again!