2001 YZ426F Jetting

[jakescott]

I'm having troubles fouling plugs on cold starts. I already did the blue wire mod, and tried leaning the fuel screw. I need good jetting specs for 2001 yz426f w/ FMF powercore IV S/A pipe. 1000ft 60degrees F. Any help is apreciated!

 

[biglou]

Good starting point would be 42 pilot, middle position on needle, and 1-1/4 out on the pilot screw. Definitely don't touch the throttle at first. If it won't fire, try one blip then kick again. Other possibilities could be valves out of adjustment.

Is this bike new to you? Any changes lately? Did it just start doing this? If it was jetted for winter, and now it's warmer, it's probably time to lean it out. I found it nearly impossible to foul a plug on my 01, however.

 

[jakescott]

Thanks for a start. I plannen on reassembling the bike today and giving it a wirl. I bought this bike brand new in 2000 as soon as the 2001 models were on the floor. Unfortunatly I've been having this problem the entire time I have owned it. I just keep pumping new spark plugs into it. Someone suggested that the accelerator pump may need to be adjusted. Has anyone ever heard of that(if so, any tips).

 

[biglou]

The accel pump only comes into play when you twist the throttle. Which you aren't normally doing when starting. Maybe a blip or two if it's relatively cold outside. There are so many possibilities as to the cause here, but starting with the basics won't hurt.

 

[Matt90GT]

First off adjust the valves. If you dont, you may be chasing the wrong problem!

Once the valves are adjusted, it is known that the AP squirt is a bit aggressive on these bikes. The BK mod is one way to solve that issue:

http://m.bobbitt.home.comcast.net/yz426fm/bkmod.html

You could alternatively install the 2002 AP diagaphram.

Personally I have a JDs Jetting kit in the bike setup as suggested in the directions, ZIpTy Fuel screw and the BK carb mod. no bog, no fouling plugs. Just sweet running!

You may have some other issue with weak spark, but it would be the last thing I would check. Valves, jetting then coil and grounds.

 

[Hick]

It is conceivable that you have a bad CDI. If you have the opportunity to do so, I would recommend you temporarily swap units w/ antother 426 owner (any model will do) to try and isolate this problem.


I'm perhaps biased towards this conclusion because I had a '95 KX that ate plugs for breakfast despite any jetting changes until I replaced the CDI.

Also, at your elevation stock jetting should not be rich enough to blame for persistent plug-fouling. But it seems that a few '01 owners have had persistent plug-fouling problems.






OTOH it could also be a bad float setting, or maybe even the AP (which is widely recognized to be overly judicious).


There is a simple mod to limit/adjust the AP operation that was designed into the '00 and later FCR carbs on these bikes. Over at Thumpertalk.com this mod became known as the "BK mod" after Tim Ferry's (then) mechanic's initials.

Just a few things to consider, hope this helps.

 

[ag9123]

2001 YZ426 jetting

I'm running the same pipe with a 42 pilot/168 main. The BK mod is a must. I would also clean the carb thoroughly and verify the float height is correct.