YZ426 off idle bog. Please help!

Long story short: A friend bought a YZ426 recently and he said it rarely starts and runs like crap when it does start :thumbsup: I offered to help tune it in for him and now I'm sorta scratching my head.... It has the power now intake mod, boyesen quick shot, and an aftermarket fuel screw to make tuning easier. A slip on spark arrested muffler and NO air box lid seems to be the only other modifications.

After starting it I found the ilde to be erratic and what seemed to be too much fuel on bottom. Not responding well to the fuel screw I pulled the carb down and found a 48 pilot which I swapped out for a 42. The bike starts decent now but it has a nasty bog when the throttle is wicked. A slight bog seems acceptable to me but this is flat out fall on your face bog while riding with a warm motor. Here's what I'm thinking please let me know if I'm on the right track here :bonk:

1. Too much fuel or too quick of a shot from the accelerator pump? Is the timing on the pump adjustable? If so how do I go about doing this?

2. Pilot jet? Seems to run and idle fine on bottom as long as I have the idle set fairly high.

3. Needle too high? Mid to top seems great though....

I pride myself in tuning 2t's, but since I've been living under a rock for half my life this new stuff is a little out of my league :ride: Anyway we're sitting right on 4500ft. here in Utah. Thanks for any tips or advice!


I forgot to add. This bike does NOT have the BK mod and the spray from the accelerator pump is nearly instant. There is almost ZERO delay from the second I snap the throttle to the time it sprays fuel from the accelerator. Is this due to the Boyesen quickshot?

So whats right and wrong here? My thinking is the pilot jet needs to be set with the fuel screw and from what I'm seeing it is and they are both working together and adjusted properly. Once I snap the throttle and an instant spray of fuel seems to be bogging the motor so I'm singling out the accelerator pump here... Thinking out loud :thumbsup:

What year 426? What needle is in it? OBEKR? OBEJP?

Bog under a load? (not on stand)

Unless something is way off, I would think a bog would be more of a lean condition than too much fuel. This is assuming the carb is clean and is assembled correctly (slide)

Reading up on the BK mod should give you a good idea on AP squirt timing and duration.

Don't forget about checking valve clearances.

BTW...Get your 500 running.....I want a full ride report on how it runs and how that shock works.

I've been reading the FAQ section in this forum all night. THANK YOU to everyone that contributed. The carb section is incredible and I've learned a lot today :bonk:

moto2000: The bike is an 01 and the Needle is an OBEJQ is that good or bad? It was set second notch down from the top....

Bog is under load during casual riding and I'd say the bike is not ride-able as bad as it is--scary!

I've been reading all night and came to the conclusion that the AP timing and duration is likely off and I'm also going to give the o-ring mod a shot.

Anyway if it wasn't for this d$#m Yammi I'd be tuning the 500 in right now! :thumbsup: LOL! I can't wait to get that one going, but I have a long road ahead as I'm starting from scratch with that PWK. I'm already sick of jetting and the season hasn't even started :ride: I'm planning on taking it on our annual West desert trip so I should have a good trip report that I'll link you to. Thanks for the help!


I would think a bog would be more of a lean condition than too much fuel.

Agreed x2

I asked about the needle because the 2000 had a pretty bad bog and a big help was going to a slightly richer needle from a 01. obekr to obejp

The 48 pilot is pretty strange. I run a 42....sometimes a 40. At your elevation, I would think a 40 or even a 38 would be the norm. There should be a ton of jetting info here for your elevation.

The AP squirt should just miss the slide when you whack the throttle open. Maybe 1/2 to 3/4 sec in duration if I remember right. I used to tune it on a bench with with a remote tank.

If all else fails and you're at your wits end, I have a extra carb you could try from a '01. Just let me know and I can ship it to you.

If all else fails and you're at your wits end, I have a extra carb you could try from a '01. Just let me know and I can ship it to you.

Thanks man I really appreciate it! Turns out it was the needle and pilot. With the needle in the 4th clip and the pilot at a 42 she runs very well. There is still a slight bog once in awhile if I really hammer down, but nothing I'd worry about even if it were my bike. For whatever reason the PO installed a really fat pilot, but leaned the needle out almost all the way :thumbsup:

Anyway thanks again,


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