YZ400 off idle bog, read all the info still ??

okay picked up my new to me 98 YZ400. when test riding the bike i noticed the bog, if you tried you could stall the motor.

Well i figured i could tune it out. Got the bike home a pulled it apart just to check everthing out. Its got a Bill's pipe and other than that it is stock (jetting, etc)

this is at sea leval to 1000'.

i replaced the acc.pump diaphram, and cleand the linkage, installed the Boyeson acc. pump cover, Zyp tye fuel mixture screw, JD jet kit, (170 main,blue needle with clip 4th down from the top, and the oring on the pump linkage.) The rest is stock. Now i did only have the fuel screw 1 1/2 turns out. I fired the bike (Don't know what all the hard starting hype is about, it starts on the second kick and i had sandles on.) and warmed it a little and the hesitation seems worse than it was before. i adjusted the fuel screw out another 1/4 of a turn and really didn't see much change.

Now i've spent the last hour and a half reading post after post about fixes for this and am still confused. There's the john-now,Bk -now ,power-now, taffy, p-38, b-52, cotton candy ....etc which is great but WHICH ONE WORKS ?!?!?

Some say it isnt the pilot,

some say its the pump timing,

some say its lean,

some say it rich,

there is a ton of info , but it would take me all summer to do all of them and test. Yes i guess i can buy a new carb (some say that the fix) but that really isnt an option right now. i know a lot of these 4 strokes do it but ive seen it fixed, at least on the KTM's , so im sure this can be solved.

Can someone please point me in the right direction ?


Do the bk mod set your ap squirt timing to .5 seconds or so and go one size richer on the pilot. thats what i did to mine. Some of these other mods do not apply to the 400/426. I did the o-ring mod on the 07 250f cause it was easier but i dont think that can be done the the 400/426's. I have a 2000 426 with almost no bog. When it is jetted right for the day and the fuel screw is set right i have no noticable bog. The reason some say it isnt the pilot is because it isn't. The ap squirt was around 2.5 seconds on my 426 which is why it was bogging. It was actually getting flooded out when i would twist the throttle hard coming out of a corner. It will still bog on the stand or in neutral if you give it a quick twist of throttle but about any 4-stroke will.

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