By
VikingTrad3r
I have an 86 yfm250 (moto4 250) i picked up.
It bogs and pops a bit out the tailpipe if you can even get it higher up in the rpm. It is pretty gutless. I have other 250’s (timberwolves) and its ultra gutless compared to them.
It has an aftermarket carb on it that has a “W” on it Its not a mikuni but it looks close
i figured jetting was off at the high end.
i had a mimuni that was very crusty, swapped the main jet into the W carb, and the needle, which was set on low slot 4 so left it there.
it actually ran much better, and cured what i thought was a problem caused by a lean condition from the W carbs main jet being too restricted when i compared to the factory mikuni
I had it out and was enjoying the upper rpms that i couldnt reach before without bog. at wot it would fall on its face low top speed.
after getting it hot, it would actually refuse to start after dying.
back to the drawing board.
when i first got it i adjusted the valves. So i retraced my work and thought maybe id overtightened. double checked, nope all good. .005 exh , .003 intake, check.
So i checked the compression.
here is the trickyness. help me understand this. I am at 3500ft asl in calgary canada
- initiate test, it goes to 75psi right away. this is low. Even at my elevation im thinking 110psi minimum
but....this thing starts no problem. even -20c it starts. makes no sense to me. so i press the starter again and it goes to 85. Then i press it again and no change. Then i turn over again and it jumps to 95. Then again...no change. Then again and it goes to 115. finally finishes at 120 and i cannot get it to go any higher than this.
ive repeated this multiple times.
i press and hold the starter...it will max at 75. Then a series of maybe 10 more quick turnovers from the starter and compression inches up to 120psi.
So, i think the low compression explains the low top end, and even the backfiring out the exhaust when hot. The exhaust is rich enough (incomplete combustion in chamber) and to ignite in the muffler.
it gets worse when hotter.
makes me think i have a crack, or something that is low down on the cylinder wall. When the piston stops just above it on the upstroke, and i then turn it over again, it adds pressure to my tester reading.
thoughts?
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