Yz400f Exhaust question

I recentally bought a fmf ti4 mid pipe and silencer. but sometimes when i let off the gas it backfires a little bit i was wondering if the bike needs to be re-jetted or if it is normal for this to happen.. thanks

Try to turn the fuel screw out a bit to richen it. If that doesn't work, you may need to rejet. No big deal. Sounds like you need to be richer.


It's probably nothing to worry about, unless it annoys you. The most common cause of this is an air leak in the exhaust system. The Ti4 (if it's like the one on my 450, which does the same thing) has no clamp at the mid pipe like the stock one does. This point is most likely where the leak is. You may want to see if the stock clamp will fit and try that.

With the throttle closed, very little of the fuel fed through the idle circuit will burn in the engine, and gets passed into the exhaust. Since it's also hot in there, you now have two of the 3 ingredients for a fire. Add a little air and you have them all.

Set your idle screw to make your engine run right, not so it sounds right going down a hill.

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