99 WR400sm bad back fire/popping

4 replies to this topic
  • sdg8569

Posted March 15, 2009 - 08:16 PM


I was wondering if someone would be able to help me with a problem Im having with my WR400. The bike had an FMF powerbomb header w/stock pipe and some sort of baffle at the end of the pipe. (Ive attached a pic with the original set up) I replaced the silencer with an FMF Titanium. Ever since I change the pipe the bike backfires or pops alot when I let of the throttle. It seems to run nicely until like I said I let of the throttle. Any suggestions? Im afraid its going to do harm to the engine. Also the pipe is load as hell. Is there any way to quite it down a bit?Posted Image

  • Birdy426

Posted March 16, 2009 - 02:30 AM


It sounds like the joint between the header and the mid-pipe is loose and leaking air.

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  • doctorchopper

Posted March 16, 2009 - 04:41 AM


It could be running a bit lean with the new pipe. As for the noise, you could try re packing it or i've use a db dawg in the past it quitens it down a bit with very minor power loss.

  • sdg8569

Posted March 16, 2009 - 04:29 PM


I used a high temp rtv on the joint between the mid pipe and silencer and I check with a tissue to see if there was any leaking and there was none.
Any more suggestions.:)

  • Krannie McKranface

Posted March 16, 2009 - 05:12 PM


"Decel-pop" is a sign of:

- lean pilot circuit
- air leak at exhaust
- air leak at intake

Check for leaks with a can of brake cleaner on the intake side.

If you went up to a FMF, you are scavanging more exhaust, which means more air is getting in too: you need to richen your pilot circuit. Try going to 2 turns out on the fuel screw. If no change, you need a bigger pilot.


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