BK MOD=Jetting help needed!!

I have just done the BK MOD on a 00' 426, problem is it would barely run after the jetting changes. The bike is completely stock, after the mod I tried these jetting changes:

1. 165 main

2. 45 Pilot

3. Raised needle(Lowered clip) one position.

4. Fuel Screw, 2 1/4 turns.

I run 93oct pump gas, always Amoco.

It ran terrible, popping and crackling. After alot of mechanicing, head scratching, and a few assorted word, I returned everything to stock. I am riding in the Atlanta,GA area. Can anyone get me a starting point?? I don't have to much knowledge of 4-stroke jetting clues, other than to trust what I feel. Thanks for the help!!!

I am at 5000 ft ASL, but its colder here. I am running:

1. 165 main at 40 degrees, 162 at 60 degrees

2. 42 Pilot

3. needle clip #2 from top.

4. Fuel Screw, 2 turns or so, depends on temp.

Mine also ran terrible on clip #4. Try #2 and get back to me. I went richer on the main after the BK mod, but richer on the needle did not work. That crackling/gurgling at 1/4 throttle was too rich on the needle.

My jetting at 40 degrees should be similar to yours at about 80 degrees.

hey just to tri and help a little with jetting if your bike is running fairly good with std jetting you may be doing to much it all maters if you have opend up the bike such as pipe air box air filter backfire screen also remember the fuel screw is just what it meens it only takes small adjustments to do alot you may only need to change the main one size and the fuel screw out 1/2 to one full turn remember to only run pump fuel in the yamaha or it will be hard to start.

remember to only run pump fuel in the yamaha or it will be hard to start.

I have run race fuel in my YZ-F's since they first came out at the end of 1997. I have NEVER had a starting problem with any of my YZ-F's. I do make sure that I have the jetting correct as that will adversly effect starting.

Thanks for the tips, I could barely get mine past 1/4 throttle. I did leave the BK screw doing its thing, although I lengthened the spray to about 1.5 sec. It seemed to run ok, with an occasional bog from a hard landing or two. Any ideas on the cause of the bog.??? One of the problems is that the area I'm in, the weather, this time of year can change daily. This week is "supposed" to be pretty stable, so we will see.


I also live in Atlanta and had the BK mod done about a month ago...runs awesome. The jetting that I'm running is 168, 45, needle moved one clip. I am running VP C12 racing fuel. The only time I get the "bog" is when my air filter is real dirty or I used to much oil. I have found you need to watch your filter more so on this bike than my 2 stroke.

Ricky at FreeWheeling Honda/Suzuki and Larry Dukes at LSD Racing helped me out...give Ricky a call and he can help you out. He is very good with 4 strokes! Good luck.

Do you do any racing?



I have had Larry do some work for me on a two stroke, I have met Ricky briefly, the whole staff out there are good people. My job keeps me from getting in to good shape, mostly ride for fun right now.

On the needle, is that going richer??

Also, probably going down to International Sunday, hear that place is a blast!!

I answered my own question shortly after typing that. 4th position is richer.

What is your temp right now? If it is 80 degrees or so, try 165/clip#2/42/2 turns with the 0.3 second duration to start off with. Note that I have an 01 and use pump gas, is the needle the same on an 00? If so, I think that will be a good starting point.

After setting the 0.3 BK mod duration, and making it just miss the slide, I would then set the pilot/fuel screw. Make sure the bike is up to operating temp, very important. If you have a tach, set the idle speed to 1800 RPM, otherwise guess! Note the idle RPM at 1/2 turn out, 1 3/4 turns out, and 3 turns out of the fuel screw. Which is fastest? If its 1 3/4, you have the right pilot. If its 3, then go up to a 45. If its 1/2, then go down to a 40. Now fine tune the fuel screw. It helps to have a tach but most people get good at it without one, to find max revs with the screw. You can also turn the fuel screw in until you get decel backfiring, then back it out 1/2 turn from there. I also recommend that Kouba fuel screw adjuster, getting a driver bit into that recess in the carb is a pain.

Next I set the main. I have a temperature vs altitude chart that works for my bike. I can send it to you. But alot of people do uphill WOT runs comparing how well the bike pulls with different jets. Do the same with the needle, looking for how it runs at 1/4 to 3/4 throttle. If you get that gurgling/cracking sound, that is too rich. I never had much luck with looking at the plug. My 250F plug looks totally different than my YZ426, yet both are jetted well.

Good luck. BTW, I had a bad problem this weekend and I could not figure out what was going on. I remembered I had drained my 5 gal gas can into the bikes tank. Water is more dense than gas, so maybe the gas can had some collected at the bottom. I drained about a cup of gas using the 3mm allen bolt on the float bowl. The bike ran great afterward.

[ November 01, 2001: Message edited by: mikeolichney ]

[ November 01, 2001: Message edited by: mikeolichney ]

brian, call Mark at 770-924-7404

my bike is there right now getting the BK mod as well as a valve adjust. He can straighten this right up for you. plan on copperhead Sat. for race. call me

BK14MX: Here's my theory, I don't think you want to run the squirt duration for 1.5 sec. Either you jet the bike for proper air/fuel mixture during the squirt (stock) or for after the squirt (BK mod). You are jetting the bike richer after the BK mod to make up for the lost fuel. You don't want it to be too rich for the first 1.5 seconds, whereas 0.3 seconds is just transition. (After all, the APJ is just there because when you whip open the throttle, it takes the jetting a little while to get gas into the air. The APJ is just giving it a boost during that quick transition). And you dont want to jet so that it is too lean after 1.5 seconds. I think the stock 2-4 seconds on a 426 is ok because not many people hold it WOT for longer than that.

mike, good point. I am going to start over with the jetting, its relatively stable around here right now, weather wise.

Hey Dave, I have to work this Saturday, but I'm going to go to International on Sunday.

I got ahold of Mark right after the I finished the mod. Im still not sure if it was too rich up at Copperhead or what, but it ran fine when I put it back to stock. I've got some good starting point info now, Mark had some very good points too.

Where can you get that Kouba fuel Screw Adjuster?? That screw is seriously a pain to get to!

Mike: That chart you have made could really come in handy, my email is BK14MX@mindspring.com.

Right now the temp is around 70 for the high, I have gotten mixed reviews about the altitude here, most tracks are between 1k-2k high I think. It will be getting cold here very soon, almost too cold for this florida boy to ride!, almost. Thanks alot for the Info.

[ November 02, 2001: Message edited by: BK14MX ]

The FS adjuster is at koubalink.com. $16.95, I think. I will email you the jetting chart, its an Excel speadsheet. I have a canned email since I got so many requests for it. It is for a stock YZF. You need to replace the red STP jetting numbers with BK mod STP jetting (jetting at 70 degrees, 0 ft ASL). I would go with 168 main, clip #3 or 4 (I don't know about 00 needle), 42 pilot. Hey, its not perfect but a good place to start your jetting, should you decide to go riding in Afganistan in December.

Didnt realize you were going to be so popular did ya?? Thanks for the help, when I get it set, I'll forward the results.

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