00 426 ?

Whats up guys?

I have had my bike now for about 6 months and I absolutly love it :lol:

I followed your instructions on how to adjust valves and maintain it and enjoy working on on it. All of the problems people talk about starting puzzles me b/c mine starts on the first kick (cold) every time and first or second when hot.

I know my packing is gone in the stock exhaust (stock muffler and aftermarket header pipe) and it pops or backfires when I let out of the gas in the high RPM range. Today I put 2 strips of duct tape ( :banghead: yea duct tape I know :applause: ) over the air box and it cured it pretty much. Of coarse I run no air box lid. So I inturprate this as meaning I'm running lean and need to re-jet. I 'm thinking 1 size bigger main and smaller needle or bigger needle jet. ...Also I was tuning my fuel/piolet screw and the darn thing fell out somewere. I put a reg. screw in (until I can get another) and dosent idle perfect but seems to run fine (maby a little slower). Anyone got any ideas?

P.S If any one on this site has not done the bk mod they need to be shot. :lol:

Anybody out there??????


First off, decel backfires are caused by either lean idle mixture, or an air leak in the exhaust, most commonly where the header joins the mid pipe or the cylinder head.

Get an extended fuel screw to replace yours, and be sure that you have one, but only one, spring, washer, and O-ring, in that order from bottom to top in the bore with the screw. Don't run the bike anymore until you get one.

And lose the duct tape. YZ's don't have or need an air box cover.

Now, why exactly should I do the BK mod?

Throttle response is greatly improved

of the bottom bog is illiminated if tuned properly

easy mod


I dont think the BK mod is needed for the 450's. Yamaha solved the bog issue with the AP diaphram by then.

Get an 2001 needle (OBEJP) for your carb and a tunable fuel screw like Grayracer mentioned. Fix any leaks and report back. If the problem still persists then start playing with the jetting.

Excue me :banghead: !

Let me re-phrase

Any 426 owners on this site who have not done the BK mod.... :banghead:

I ordered the zip-ty piolet screw today and other parts from dealer also.

Thanks guys!

Throttle response is greatly improved

of the bottom

In order for the throttle on my 450 to respond any better it would have to react to my thoughts. :banghead:
In order for the throttle on my 450 to respond any better it would have to react to my thoughts. :banghead:

You mean it doesn't already? Mine does....but I must stutter when I think!

LMAO :banghead:

Checked the pipe again (b/c it started rattling) and found where the pipe meets the cannister/muffler, there was a crack in the weld. Drilled the riviots and also found the baffle (metal mesh in center) broken in half! Had a friend weld ot all back up and re-pack it. Thanks Travis at Lester Performance! It runs great again. Still waiting on fuel screw though. Thanks for the help!

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