Rear main seal???

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?



Re: Rear main seal???

Andy Kochvar /

I am actually in the process of replacing the crankcase seal right now, and check out the engine bearings. When I try to start my 'ped no matter how hot or cold it is, it's a pain in the ass. And it floods each time after i run it a few miles, and i think it's because the crankcase is filling up with gas/oil because it's a continuos leak at the crankcase gasket, and unleashes all that fluid into the cylinder at times. (my theory) When I took off my base cylinder there was ALOT of fluid behind it, when there isn't supposed to be. Big leak. It is going to be time consuming, but it's worth it to have a more reliable moped.

Re: Rear main seal???

Masa Toiya /

Let me know how the seal replacement goes. In the meantime, I will work on other possibilities. The muffler didn't seem clogged and removing it just made it loud. Timing is correct. Spark plug is new. Carb is clean. The problem is definitely temperature related. Any suggestions? Anyone?


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