My Jawa runs great when it is cold out (less than 50 degrees), and runs really lousy when hot. I noticed this in spring when it ran great on cold days and not so well on warm days. It seems to be running a bit rich, but I have opened up the air intake a little bit already with no improvement. I lowered the needle by one notch but that didn't do anything. When hot it is really hard to start, and has no power. It gets worse as the engine warms up.
On semi-warm days it feels as if the engine "catches" a power band and starts to accelerate smoothly, but after a few seconds it starts to buzz and vibrate and slow down. Sounds like 4-stroking but removing the air filter to lean out the mixture makes it worse.
Someone suggested a leaking rear main seal (crankcase seal?) but I am not sure why that has an effect only when hot. Also, replacing the seal is going to be a time consuming job, if I can find a replacement for Jawa at all.
What do you guys think?