05 WR450 Backfiring

My 2005 WR450 runs well, but over the last year it keeps mildly-backfiring on downhills or decelerating. Have had valves retimed, air filter cleaned and recleaned, plug changed, etc. Bike runs really well but keeps having this same problem.

I asked mechanic if FMF Q2 might need to be repacked, and they said that wouldn't cause the problem.

Any other ideas from the Yamaha Dream Team?

Have you tried richening the mixture at idle (fuel screw)?

You might be getting a bit of a air leak in your exhaust.

You could try removing the header, and re-installing, using some silicone, and torquing to spec....

Another possibility could be the pilot jet getting gummed up. Try replacing it.

If it seems to be temp, or weather related it could simply need a bit of a fuel screw adjustment...

Did it just start out of the blue?

Is it consistent, or worse some days compared to others?

How long have you had the bike? If it does it in the winter, but not summer for example, I'd say you just need a bit of a jetting tweak.

The two main causes are cold are entering the exhaust or a lean condition. If the exhaust system is tight and the fuel screw is set correctly it may be time to re-jet.

Did you check the ACV? It adds fuel when coasting to eliminate decel popping. If it is disabled or not working right that can make a big difference. That and the exhaust leaks are the most common causes on a WR (Along with an open muffler). Gummed jets (not clogged, just a layer of varnish) manifest in hard starting and poor idle.

Do not arbitrarily put in a larger pilot or open the fuel screw. That would make hot starts a nightmare.

Thanks everyone for the suggestions. It will take me some time to work through everyone's ideas, but you've given me some great focus areas!

It could be the AIS, if it hasn't been removed

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