Hello I have a 1985 Yamaha YFM200. I just finished rebuilding the engine and have about 125 psi compression, spark, and I don’t think the carb is the problem. After putting it all back together I can’t get the thing to start. I can get it to start but it only runs with the starter going. Sometimes it will stay running by it’s self but I can still hear the starter spinning and I can’t rev the engine at all. Please help!

 

If the starter continues to run when you take your finger off the start button you have a big problem, but concerning the non running engine. Check the piston is at TDC when the T mark says it is. If timing is OK, it has to be a carb problem, take it off and clean the jets out. Check the rubber inlet manifold is not cracked, and try again.