This is driving me crazy!!!

First off, I am sorry for asking another jetting question, but I just can't help it. I have an '02 426 and I fouled a few plugs in it when I first got it but none since. But, every time I pull the plug out it is very, very black and sooty. My bike seems to run fine except for the "YZ Stumble". When I got it it would puff a little black smoke when it stumble so I turned the fuel screw in, and then in some more, and then in some more. I that now I am only like 1/2 a turn out and it still has the stumble most of the time. I also can never get it to pop on deceleration. I was sure that going down to a forty on the pilot jet would solve the problem but after reading on TT for 2 hours today and reading about people running 48's at about the same altitude and climate I'm a little confused. I live in the desert of southern Idaho where it is about 85-100 degrees while I am riding, about 20% humidity, and I think that the elevation is about 3000' but I'm not real sure on that. Should I put the 40 in and fine tune it with the fuel screw or am I going the wrong way? Is this all I need to change on my carb, or should I consider main and needle postions as well? If any other Idaho riders could tell me what jetting you are running it would be great.


Try dropping the needle to postion 2. Second from the top. Most people will tell ya that it shouldnt effect it but it will. and is an easy change.

Anybody who is running a 48 pilot at 3,000 ft. of elevation either has:

a) A 400

:) A 426 with an overly rich pilot.

I agree w/ Shawn and would try it a clip leaner first, then try the 40 pilot (then reset the idle mix). I'm thinking your bog may be a rich spot as you come off the pilot onto the needle. Being overly rich, or lean for that matter, on the clip position has the most effect if you REALLY whack it open.

Smaller throttle openings are governed by pilot and also needle straight diameter, but stock size on the latter should be good, or very close, for you unless you are a bit higher up than 3k ft.

But try one clip at a time, stock should be #4 (from top down), go to #3 and then ride it. Make sure the bike is fully warmed up before you decide if it is better or worse, and only make one change at a time.

Hope this helps.

There should be no "YZ stumble" on an '02 bike. 42 pilot is stock, higher altitudes require leaner jetting. I would follow hicks advice. If you are only at a 1/2 turn I would definitely try the smaller pilot along with the change in clip position.

I have a stock '01 and am running a 40 pilot at one leaner clip position than stock (I forget if that is 2 or 3) here at sea level and about 85-95 degrees, high humidity.

[ July 23, 2002: Message edited by: skthom2320 ]

I ride around 4000-6000 and experience the stumble when whacked open from a low idle. I have stock jetting right now, should I go up 1 clip leaner on the needle, I also get the decel popping so my idle jet must be lean enough right?

Just to through in a monkey wrench.

There have been reports of bad CDI units, and once replaced the fouling stopped

I also have a 02. I did the BK mod and at the same time, went to a leaner needle (recommended by the Dr. himself), bike runs like a dream. alot of folks don't think the needle has much inpact. It's really easy to change, 10-15 min. Try it and let us know what you find out

Side note, if it ever puffs black smoke its too rich, by quite a bit really. My tail pipe is a nice clean gray. Yours should be too.

Drop to the smaller pilot and drop the needle 1 postion. Then ride it a bit. Should be a lot crisper. Then clean out the end of the exhaust pipe. Then run it wide open for a bit, down a road or something, somewhere you can really ring it out. IF you main jet is right, it will color up with a light gray. It will probably be too rich and be black. Black is safe, but slow.

Alright, thanks guys I will go ahead and drop the needle one postion right now and get the 40 pilot when I get my new oil filter in a day or two. Just wish I could try it tonight but I have no oil filter to go back in it and the oil is already drained.

Something I learned the hard way. Before you mess with the jetting take the time to be sure the carb is clean BEFORE you play with the jetting!!

Well I just moved the needle clip up one position but I am won;t be able to see how well it works until tomorrow night when I get my new filter. And Thursday night is track night so hopefully I will no longer have to worry about this as I am rolling through those blasted whoops.

Thanks for all the help guys, I finally got my new oil filter and just rode up and down the lane for few minutes but my bike started and ran much, much better than before. I still might end up switching to the #40 pilot but for now just the changing the needle clip one postion was a major improvement.

Only one comment needed to add here "black is safe"

This is only to a point. Over fueling will cause "fuel wash" whereby the lubrication is washed from

the cylinder walls.

In other words, jet it properly or pay.

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