Back Firing

Just put my stock pipe back on yesterday '99 yz400(had a FMF Powercore IV on there before) and it's now back firing a fair amount. Any suggestions to relieve this problem?

Back-firing is usually due to a lean mixture. Did you change the jetting after the putting on the other pipe?

I rode a friend's 426 (un-modified and original exhaust system) yesterday, and I noticed that it made a bit of "snap-crackle-pop" on the over-run. Is it possible that you forgot that it did that?

Left jetting the same, my guess is that it's a little more than snap crackle pop. Which way do I scew the jet to make it less lean?

Could also be due to leaks in the intake or exhaust side.

Don't know about your bike, but usually, screwing clockwise blocks air and makes it richer. However this will affect the low throttle openings. Jetting affects the higher throttle openings.

I think you should try and find what is causing it rather than try to cover the symptom - it didn't do it before and you say you haven't changed anything, so you might have something which is not right.

Maybe the stock pipe is less restictive than the FMF thereby creating a lean condition. Try turning the fuel screw out a 1/2 turn or so. (is there a fuel screw on a 400???)

Id bet the head pipe isnt seated. You really dont have to mess with the jetting going from the stock pipe to the FMF.

My other reason for thinking this was, the bike didnt do this prior messing with the pipe correct?

Id loosen the entire pipe front to back, and resnug the header as evenly as possible until its good and tight, then tighten up the rest of the pipe.

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