WR 400 backfires (??)


After installing new piston, boring the cylinder and reinserting the camshafts the engine seems to bacfire or something??.

i start the engine, it sounds good for 10-15 seconds at idle speed, then it "pops", and dies. the sound is comming somewhere from the air filter region, and i can feel the "pop" if i put my hand on the air cleaner joint.

it does this everytime. the larger the RPM, the louder the "pop".

i have not messed with the jets, other than the idle speed screw. im not sure if this is your ordinaty backfire, my experience of backfireing is that you hear a larger explotion from the exhaustpipe and the engine keep running. now for me, the sound is not so large and doesnt come from the exhasutpipe but from the aircleaner joint as i said, plus the engine always dies.

im not 100% that my camshafts is correct aligned, as i have installing them myself and im novice. they could be off 1 pin. read more about my camshaft installation (and why it could be wrong) here:


could this be the problem?

when i run the engine at low idle speed, this occurs after 10-20 seconds. if i turn the idle screw up so the idle speed is slightly higher (higher than "normal" idle speed) the engine keeps going very nice, but when i open the throttle after a few tweaks it stops, same thing as before. (a "poff" sound, and you can feel it near the air filter region) then engine stops imediately.

My wr does the same thing if it is not warmed up.Especially Right off the crack of the throttle at cold temp.Try warming it up to operating temp.see if it stiil does it.You might have to play with your jetting a little. :D:)

Sounds like a lean backfire, or your exhaust cam is retarded :)

retarded = late?

should i take it out from the chain and put it back 1 pin counterclockwise?

:confused:Time the cams by the manual. If it was too retarded you would have bent valves by now :)

if i had bent valves, why does it run good 15-20 sec then suddenly stop with a poff?

hm. now when i have turned the idle speed up some, the problem is no more.

funny, yesterday when i did the same thing, it died when i pulled the gas with a louder pop. oh well

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