QuestionI have a 1975 Honda XL350 that always runs rich. I have rebuilt the carb, and now have a replacement carb on it. I dropped the jet needle all the way down to lean out the mixture. I have all new ignition and the bike is timed correctly. The bike starts on the first kick and warms up very quickly. The bike has 180 psi compression but smokes quite a bit. The bike runs good for a few minutes then it starts to chug. The spark plug is always black and wet looking. My guess is that the oil rings are no good, but he scraper rings are still good. Any ideas ?
AnswerGreg, you mention smoking all the time and black oily plug, which along with high compression (carbon buildup) indicates that the engine is using excessive amounts of oil, which could be from rings and/or valve stem seals or worn guides. Oil smoke is blue, whereas excessive fuel is black smoke coming out the exhaust. Do you have a lot of blowby coming out of the breather tube?
Dropping the needle position will affect 1/4 to 3/4 throttle settings, with the main jet size affecting the whole range, including top end fuel ratios.
Be sure that the spark timing is correct at idle, then verify that the spark advancer is moving back and forth normally, as well.
After 32 years, it is probably ready for a freshening up on the top end. If it sat for a long time, some of the rings could be stuck in the ring grooves. If the piston was ever seized, then the rings can't seal correctly against grooved cylinder walls, so they use a lot of oil.
Bill Silver
