hi all, i just picked up a 89 yfm 250 moto-4. it will start (just barely) but there is some back firing going on through the carb. i suspect the timing chain is probl. stretched over the yrs. does anyone know if this chain is adjustable? if so, i sure could use some guidance as i've never done this before. any help would be greatly appreciated!

 

youll notice this little thing sticking out of the side of the cylinder, and its a timing chain adjuster that automatically adjusts the timing chain, the timing chain is in between these two guides and the adjuster automatically pushes on one of those guides to adust the slack, so if theres still to much slack then either the chain and or the guides are worn out.


Number 10 is the tensioner, its pushes number 9 which pushes against the timing chain.

 

is there a way i can visually check the chain or tensioner while its installed? should there be much play in the chain?

 

if the tensioner is working right then it shouldnt be loose unless its worn out, usually you can hear a difference when there worn out, you can pull the cam cover off and try to pull up on the chain and should be pretty snug.

 

thanks for the info, i'll try it first thing in the morning. btw, do you know where i could pick up a pretty good service manual for this thing? it would be a huge help to look at pics while i'm doing all this. thanks again for your quick response.

 

any yamaha dealer should have one, or some ebay stores carry them also bikebandit.com and here-> Clymer Yamaha ATV YFM250 Timberwolf Shop Service Repair Manual i know its got a timberwolf on the cover but the manual covers 89-91 yfm 250 moto-4s also.

 

just ordered it, thanks. one more question, lets just say something IS wore out, and i have to replace either the chain or tensioner. is it a big job? a dyi job? i can get around most things like this, just never done a 4 stroke atv before. lol

 

it kinda is a big job,you gotta set the timing to TDC then you gotta take the head off to get the tensioners and chain out, and youll need a new head gasket and side cover gasket since you gotta pull those off, your problem might not even be the timing, it could just need the carb cleaned out.

 

that very well may be, but i've never seen a carb cause a backfire. i've seen carbs do alot of weird things before, but never saw one cause a backfire. i did remove the carb today, tear it down and clean it (carb and choke cleaner and 150psi of air do the trick pretty well). i didnt see any major issues with that. but like i said, im not an atv mech. by any means.

 

did you check to see if the timing marks lined up? you should be able to line the mark on the cam sprocket to with a mark inside the cylinder head not far from the cam gear itself, then line the crank up also, there should be timing hole with a plastic plug on the left side case that u remove to align the timing marks.