2000 yz426/ boysen accelerator pump cover

i just bought a used yz426f and i love it. suspension is awsome for trail riding and it handles well the only compliant i have about the bike is the notoriious low end flat spot in the powerband that i have heard most early 426's have. The bike runs fine otherwise. i work on bikes so i have tried tuning the carb a bit which has helped a bit but there still is that little hesitation in the powerband. i saw an advertisment for a Boysen accelerator pump cover that is is supposed to help the low end bog. But my first qustion is that is this pump cover really work? if anyone has used this upgrade any info will be appreciated. thanx

probikerguy2,

Have you done the BK mod ?.... this helps a lot !

Stonie426

Where can I get a link to the BK mod? I used to be able to find it, now I can't.

i just bought a used yz426f and i love it. suspension is awsome for trail riding and it handles well the only compliant i have about the bike is the notoriious low end flat spot in the powerband that i have heard most early 426's have. The bike runs fine otherwise. i work on bikes so i have tried tuning the carb a bit which has helped a bit but there still is that little hesitation in the powerband. i saw an advertisment for a Boysen accelerator pump cover that is is supposed to help the low end bog. But my first qustion is that is this pump cover really work? if anyone has used this upgrade any info will be appreciated. thanx

It works as advertised. I could really tell a difference in mine, it redefines the phrase "throttle response".

It works as advertised. I could really tell a difference in mine, it redefines the phrase "throttle response".

i know mines a 450 but i'm with 642mx :applause:

hey what does the BK mod do ????

It allows you to correctly adjust the accelerator pump "squirt" of fuel. My bike came from the factory with way too much fuel squirt and way too soon. You'd snap the throttle open and could watch the acell. pump charge hit the back of the slide instead of just going under it into the cyl. The BK mod made a noticeable difference over stock and I think is named after Tim Ferry's mechanic who came up with the mod. Garrett Berg was the one responsible for bringing to TT I believe.

I have not tried the Boyesen mod yet. please let us know what you think.

It allows you to correctly adjust the accelerator pump "squirt" of fuel. My bike came from the factory with way too much fuel squirt and way too soon. You'd snap the throttle open and could watch the acell. pump charge hit the back of the slide instead of just going under it into the cyl. The BK mod made a noticeable difference over stock and I think is named after Tim Ferry's mechanic who came up with the mod. Garrett Berg was the one responsible for bringing to TT I believe.

I have not tried the Boyesen mod yet. please let us know what you think.

thanks dude

You really should do the BK mod before you go for the pump cover. My bike hasnt bogged since.

i was going to purchase the boysen cover.....when i went to go get it i found out about the factory r $ d p-38 lightning it does the same thing in addition it has a optimized shot for your specific bike. it also comes with the p-51a air/fuel adjuster for the same price. i chose the factory r $ d part. i have installed these parts today and i will be testing them tomorrow. i will post my results. i think those guys down there are into old war planes....

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