Odd Jetting for 6000'

Have a new 01 426 and am using stock jetting here a Colorado Springs (~6000', 75deg). Have been getting decel popping (sometimes) and studder/hesitation from 0-1/4 turn of the throttle (often, usually after a jump and coming back on the gas). I assumed it would be running rich due to the sea level jetting. Pulled the plug and found black threads and white electrode (lean). A little puzzling. Stock main, stock clip position (4) and stock pilot. Almost stock 1.5 turns out on the fuel screw.

I see jetting suggestions all over this site but not always clear on the users altitude (not attitude) and temperature (not temper).

O' wise ones, will I be in the ball park with just turning out the fuel screw another 1/2 turn? If I'm lean at 6000', it would seem sea level jetting is way off base.

It appears that there is problem there. It should not run lean at that altitude with the stock jetting. It sounds like there must be something mis-adjusted.

Have you checked the jets for obstructions?

Did you oil the air filter?

What kind of fuel are you using?

Have you contacted your dealer and asked their opinion?

I live and ride from 100' to 3500' elevation and I had to lean out my 01 426 in the mid and richen the top. I have the stock pilot, 3rd clip on the needle and went up one on the main. I use either VP Red or Union 76 Red race fuels.

Your problem sounds very odd and quite different from the other YZ426 jetting issues. Most of the 426 problems are from a to rich condition. At your altitude you should already be running rich.

Not much help for you on this but again, very different from the carb issues here. There are a few guys on this site that live around your area and they might be more of a help to you.


My plug looks just like that no matter what jetting I run! I am at 3000', mid 80's temps. I am currently running a 168 main, 45 pilot (1 1/4 turns out) and the next leaner needle with the clip in the 3rd position from the top. Runs great but the plug always looks like hell!

I'll go out today and play with the fuel screw to get rid of the decel-pop and hesitation. Looks like many people are using a larger pilot than the stock 42 and 3rd clip (leaner) on the needle. Guess I don't care what the plug looks like long as it runs good and no fouling or excessive lean.

Something else. I noticed air pressure buildup in the fuel tank. When I unscrewed the gas cap the pressure would release and the bike would run better for a lap or two until the pressure built up again. Asked the shop about it and they said that pressure build up is normal. I've never recalled any of my other bikes doing it. Never had that one-way valve in the cap breather hose before either. Maybe I'll take it off. Seem like pressure would raise the fuel level in the carb float bowl and vent gas.


Make sure the vent hose valve was installed correctly. If installed upside down it would do that too.


Turned the fuel/pilot screw out to 2.2 turns and it runs fine with no decel pop and no hesitation. Also have eliminated the fuel tank pressure problem. Took apart the fuel cap and reassembled and now it seems to vent the pressure off. The one-way valve was installed in the correct direction. Maybe things were just sticky/stuck.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now