Please help me "dial-in" jetting.

The pilot is probably a little lean (the popping on decel) and the black smoke and bog usually come from the momentary rich condition generated by the accelerator pump. Do a search for BK mod, it's a quick/cheap mod that allows you to tune the accelerator pump duration to eliminate the bog (and probably the black smoke puff).

Be careful with richening the pilot circuit though, especially if the popping only occurs at high revs (as you're decelerating). If you make it rich enough that the high rev decel doesn't pop at all, it may be too rich at idle and during slow rev acceleration.

One of the common comments on this board is the stock jetting is too lean. It may not be at your altitude, but it wouldnt' hurt to try going up 1-2 sizes on your main jet and see if it pulls any harder up top. Thankfully main and pilot jets are cheap and relatively easy to get to on this bike. My 2000 came with a 162 stock but also came with a 158 and 168 main jets.

Be sure to write down the env. conditions (temp, humidity, elevation) and the jets you try each time. That way you'll build a little database about what works best for your bike under what conditions. Also, try doing only 1 thing at a time, that way the different changes don't get convoluted.

i Think if your trying to get rid of the slight popping during engine decel your chasing your tail. The conditions your discribing appear normal. If you attempt to richen the pilot system to eliminate engine braking popping your next post will be about fouling plugs. If your comparing throttle hesitation to a 2 stroke then you will always find a very slight hesitation. That is part of the beast, the combustion stroke is every other stroke compared to a 2 stroke. :)

I don't think there is agreement about them being shipped to lean. There seem to be polar trains of thought, one goes way leaner, the other way richer.


I have an '02 YZ426. Completely stock. I would like to make sure its running to its full potential as far as jetting goes.

Currently I have a popping, light backfiring when engine braking. - Throttle fully closed.

The bike has tons of power and starts easily, even without using the hotstart. Throttle response is crisp. If I quickly crack the throttle from Idle there is very, very slight hesitation and there is also a very slight bit of black exhaust smoke. Otherwise I seems perfect.

Bike is only 4 hours old. Using 4R oil 15-50.

Live in Nevada. aprox 4500 - 7000ft riding elevation. mild temperatures - 50-70F

What worries me most is the exhaust popping, the performance sem to be awesome, but I wouldn't know any better if it wasn't as this is my first MX bike. Again, everything is completely stock, including the jetting.

Sounds like you need to 1st, lean out your air/fuel Pilot screw about a 16th of a turn. It is running rich. Then see how the jetting goes....

According to this board though. A backfiring on closed throttle is caused by a Lean idle mixture. So which one is it lean (backfiring) or Rich (very small bit of black smoke)????

From my own personal experience in dialing in the jetting of this machine, a little popping. . . or, even better yet, a little crackling on decel is desirable. Why? Just as you describe, you get crisp response when whacking the throttle open right off idle. One loud pop followed by another and then another on decel is a slightly lean condition in the pilot circuit....usually. Personally, I think that your jetting is correct. As an aside, I went to Bayside, Texas this past Saturday to watch the season opener of the dirt track series. The field was totally thumpers except for one old vintage 2 stroke. I noticed that these bikes were set up to pop a little when the throttle was chopped. The field was made up of several Honda 450F's, 426's, Rotax's, CCM's, one Suzuki DRZ, and the winning VOR ridden by Chris Carr. Yep. A VOR.

Bike is only 4 hours old. Using 4R oil 15-50.

You might consider using non-synthetic 4 (not 4R) until you get 10-20 hours in it. This will help break-in everything properly. Then switch to the synthetic.

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