hi all, i recently rebuilt the carb on my 89 yamaha yfm250 moto-4. i double checked the float level to factory specs, replaced all the jets ( a pretty typical rebuild) and gaskets. the thing is runs great at an idle and for the most part throughout the entire throttle range. however, there are times it leans out to the point of dying, other times it leans out to where it will only idle. it'll do this for 30 seconds and IF it doesnt die, it'll start running fine again. its as if the float bowl is running out of fuel. i noticed this afternoon if i stand beside the 4 wheeler and shake it side to side, fuel comes out the over flow hole in the carb, then it wont run right for the next 30 seconds or so.

so after saying all that, any idea's? i thought the rebuild kit would fix the issue and it hasnt, i also replaced the fuel **** on the tank as the old one was damaged thinking maybe it wasnt in the full open position. that didnt help any either.

as always, any advice will be greatly appreciated.

 

check for "air leaks" at the carb boot/s. This still sounds like you missed something in the carb though, and it's jumping around in a spot you can't see.

But check for leaks around the mounts/boots and see if you have a crack that's developed, or maybe a boot that didn't get quite in the right place.

 

i would agree, i think i've missed something in the carb rebuild. i would think if it were an air leak it would idle and run rough through the entire throttle range, but mainly at lower rpms. i cleaned out (actually blew out with carb cleaner) all the passages during the rebuild. the book calls for the jet screw to be at 1 3/4 turns (going by memory here) out for the initial setting. maybe it really needs to be out more turns? i'll try at this afternoon.

 

Did you ever figure it out? I have taken mine apart 3 times now about to be 4. I can't get it to start. Every now and then it will turn over for like 10 seconds!