rm 125 W. back fired then quit running, power valve?

My sons bike back fired and quit running.. Spark ye.... bright blue...compression yes ...fuel yes... won't start with eather or fuel in cyclender, just back fire on 5th or 6th kick. will not roll start, just found left exhaust/power valve broke and jammed in slot. could that be the problem?

Power Valve question.... not unless it is ramming the piston and rings.  It may not run well with a stuck PV but it would probably start if that was the only problem.  Either way you will want that fixed.


1) How much Compression?  Not a yes or no type of question... bikes with "some" compression won't start well either.  Mine did that.


2) Is it sparking at the right moment?  If the timing is off that would make it difficult to start, if possible at all.


3) Fuel...yes, the carb is working correctl?  You are fouling plugs because so much gas is going through the bike when you kick it?  speaking of....


4) How about a fresh plug? 


Just trouble shooting.


If it is low compression, have you tried bump starting it?  Get that piston spinning fast enough the rings can't bleed off the compression fast enough and it fires... long hill time. :)

compression is 110 on 4th kick. timming is right on mark. now a case bolt came out at some time and chewed up the metal pick up a tad. it is hard to find a 98 flywheel used. and i dont want to pay 300 for one if that is not the problem. On ebay sellers say 96-2004 for a flywheel but WRONG on the 98 the pick up is right above the key way and all others, if key way is at 12:00 the pick up is at  8:00. i have run 100 yards with that bike and no bump start. new plug no fouling and jetting is right because it was running and then BOOM a back fire and done. piston looks good through exhaust ports. wonder though if ring got a nick when valve broke?

I have read so many dang post on this subject and no one ever comes back and says what fiexed the problem!! flustered!!!

I would say the powervalve nicked the ring. Had a few powervalve pivot pins break and always allowed the blade to go further forward than it should. But still I've had a bike start on the kickstarter with 110 lbs of compression. Check to make sure there is no debris on the magnetic pickup.

I know you said the timing is on, but did you pull the flywheel and look for a sheared key?

flywheel is clean. the only thing would be the first 1/8" of the metal pick up is chewed up from the bolt. I ordered new valve ring and gasket kit. already put new coil and spark plug wire on. next maybe new stator. the glue on a couple of the windings looks burnt but oms was right on..ring is of on this picture it was jammed in slot


That ring can't be free is it? Also I like to chamfer out those ex bridge holes, I feel the chamfer holds oil better.

no it wasn't free it was jammed under ding. believe it or not cylinder was not scratched. thanks for the oiling idea will do.

onus tested right on but I do not like the looks of stator.. the burnt glue. would this be wise to spend $$$ on new one??


well the power valve broke and hit the piston. which pinched the Ring, New ring new gasket kit... All is good thanks for all your help.

Good luck at least it wasn't too bad.

Edited by JBFL

gaskets power valve...(which I think was made of gold) and a weisco ring!

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