hi guys I am new to posting and asking questions but not real new to the site. with that being said, I just bought a 2000 Yamaha YZ426f and when I got the bike the breather box was opened up and I had assumed that they had re-jetted the carb as well but I was wrong. the bike was running way too lean and back firing when I let out of the throttle. I re-jetted the carb taking out the stock 42 and 162 jets and replaced them with 48 and 170. now the bike runs good at idle and above 2/3 throttle but when held in any gear around 1/4 to 1/3 throttle it sounds like it misses every now and then also still has a little popping noises when let off the throttle but not a backfire. Is my bike still running lean and if so should I go to a bigger pilot jet for the lower idle?

Both your main and pilot are likely to be very slightly too big already, but not much.  a 45/165 or 45/168 combo is probably more correct, but that depends on a lot of things. 


The "breather box", which I assume is the air box for the filter, only has a lid on it on the WR models, so unless by, "opened up", you mean more than the fact that the top is open, there's been no modification there. 


Popping and crackling on decel is normal for the big YZF, and it it is jetted correctly, it actually should pop "some".  By that, I mean that it will do it more on cold days than on hot ones, not necessarily every time you slow down, and not loud, continuous backfire all the way to a stop, just "some".   You should also be aware that the popping can and often is caused by an air leak in the exhaust system, either at the head, or at the joint to the mid pipe.  Jetting won't fix that.


The pilot jet controls idle to 1/8 throttle, and aids in the "hand-off" to the main circuit.  The main jet, in and of itself, controls full throttle.  If you are looking at a misfire at 1/4-1/3, neither of these is where the problem is.  If it's a jetting issue at all, raising the needle one step (by lowering the clip on the needle) should be what's needed.   That complaint does also happen when the throttle position switch goes bad or gets miscalibrated.  Try unplugging it and see if it improves things.


One more thing: "Lean" is a good thing in four strokes.  For one thing, a 4T will get so lean that it runs really bad long before it gets lean enough to hurt itself, and it will run bad enough to get your attention before that.  For another, running too rich (which a 4T will also tolerate better than a two-stroke) can cause early cylinder wear, oil contamination to the point of degrading the oil, fouled plugs, excess carbon, among other things.  The bike will make better power, run cleaner, and stall less if it's 10% lean than if it's 10% rich. 



Thank you for the info grayracer I will give it a try today and see if it improves any. Also do you think that my valves may need to be adjusted?

Won't know unless you check. 

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