I have an '88 250R that I can't get to run right. It seems to sputter and miss in the mid range and top end. I replaced the piston and rings over the winter hoping to get it to run better, but it hasn't helped. I was told by the former owner that the bike was used for ice racing but I am using it for MX. The rod has been changed and the piston height modified from stock. The bike has a 39mm Keihin carb with a 182 main jet and the clip for the needle is in the bottom postion (lean). The plug reads normal after it has been run.
Does anyone have any suggestions? Is it possible that the needle is not the correct needle? Is there a way to tell what needle is in the carb?

 

If he used it for ice racing, the main jet is probabally too big thus it is running rich. Try a smaller main jet.
If the clip is on the bottom position, it is all the way rich, not lean. This means the needle is all the way up out of where it seats and lets the most fuel in(I think). Also, there should be a number stamped onto the needle.
Try http://www.carbparts.com/.

 

Try reading the article on www.bombardierds650.com regarding carb adjustments and plug reading.

Very informative!

 

I would try the jet first. I'm not sure that's your problem because I run a 39PWK with a 195 and it is fine I jet up to 200 in the winter. The bike is very modified, but would assume that if you have a carb and pipe on your bike, you wouldn't have a problem.

Try the jet first. If it's not the jet, then you're having the same problem as a lot of the 85 ATC and 86 ATC/TRX 250R motors. Among my friends, we had 3 stators go on our bikes. I guess they start to go when they get old.

Assuming thet the bike was ice raced, the jet may be fat for the cold weather, so try a 180, or 175 to see what that does first.

ALSO, if the "e" clip is in the lowest position, it is rich, because the needle is heald higher in the jet at idle. That shouldn't affect your mid-top end.

86 ATC250R, FTZ port, polish, and decked. 89 style Pro-Series (long rod) with an 86 style .50 over Pro-X, Carbon fiber reeds on a Rad Valve and 38mm Mikuni on Alcohol machined to 39mm. Paul Turner Hi-Rev and FatBoy, PVL digital ignition (no flywheel), K&N, FTZ dyno head. etc, etc.

85 ATC250R, MT Performance (Pro-Motion engines) Port and deck, Boyessen reeds, 1.20 over Wiseco Pro-lite piston, DG pipe, 39PWK carb, K&N, etc, etc.

 

Rejet your main first. 3/4 throttle to full throttle. Go down one jet size at a time until the sputtering stops. Then rejet your needle. 1/4 throttle to 3/4 throttle. Move the clip position UP one at a time until the sputtering stops. Once they are set your dialed in. Start with a fresh plug and clean air filter. Check your plug after each jet change. Good luck!