BK Mod Results

9 replies to this topic
  • Thumpin_426

Posted September 15, 2001 - 12:12 PM


At first it was a little shaky. I had a bad bog the first 1/8 throttle. I tried tunning the fuel screw from 1 1/4 - 2 1/5 turns out and that did'nt really make much difference so I put it back to 1 3/4 turns out. I then started turning the the Squirt timming screw 1/4 turn at a time clockwise. The bog went away. I guess I had the spray timed too close to the slide going up.

I can't beleive the difference this $5 mod made in the performance of this bike. The front tire comes up so much easier and is much easier to control wheelies. The throttle is very consistant and you reach the limiter much quicker.

One question I have is regarding the spark plug color. Mine is a very light tan, almost a lean look. I think I read this is normal for this mod but is it safe?

Here is what I ended up with:

Spray duration= .3-.5 seconds
Squirt timming= Squirt just missing the slide + 1/2 turn clockwise to get rid of the bottom end bog.
Main Jet= Up from a 162 to a 165
Pilot Jet= 42 (factory)
Fuel Screw= Out 1 3/4 turns
Exhaust= FMF Power Core IV muffler with factory Head Pipe
Fuel= 50/50 mix, Amaco 93 octane with CAM2 110 octane race fuel
Rear Sprocket= 51 tooth
Front Sprocket= 14 tooth

I recommend this to ALL who always wants more and more and more........

Race results:

I raced a worked 2000 CR250 today from start to top end and SMOKED it!! Before we were pretty even up to the top of my 3rd gear and then I would start to pull on 4th and 5th. Today, he was NEVER beside me!! Two thumbs up and thanks to ALL that gave input.


  • YZThump

Posted September 15, 2001 - 01:20 PM


Hey Thumpin,glad to hear ya like the mod!!! I'm going to try the adjusting the spray 1/2 turn clockwise to get rid of all bog!! I see your in West Palm also,do you know of any practice area's close?

  • Thumpin_426

Posted September 15, 2001 - 02:13 PM


Okeechobee is the only place to practice LAGALLY. As for awesome places to ride, refer to the "Woods riding in South Florida" that posted last week. I've been riding these places for a long time and have NEVER been bothered by the cops. Just use common sense. Don't park near the entrances to these places and stay away from the residential area's. The area's ARE posted but to ride down here anymore, chances must be taken!! We're talking about thousands of acres. There's plenty of places to be incogneeto.

Have fun!!

  • sirthumpalot

Posted September 15, 2001 - 03:44 PM


Hey Thumpin, what year is your YZ? I've got the '01 and though I don't have any bog I'm tempted to try this mod to see what the results are.

  • Thumpin_426

Posted September 15, 2001 - 04:36 PM


It's an '01 as well. I did'nt do it because of problems I was having. The bike already ran like a raped date. I just did the mod because I wanted more performance and believe me, you would'nt believe the difference it makes. Put it this way, I use to race a 500hp mustang on the 1/4 mile track. I got use to the speed and wanted to go faster so I bolted on another 150hp of nitrous. I went from running 11.30's at 125mph to 10.70's at high 130mph's!! This is what it felt like the MK mod did for my bike!! Don't get me wrong, I don't think the MK mod gives you more horse power, it just utilizes what you have MUCH better. I'm telling you, do it and you won't believe the difference. I don't know why Yamaha doesn't put them on the show room this way. I think if they did, there would be no way the 250 two strokers could keep up.

Good luck!!

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  • mikeolichney

Posted September 17, 2001 - 04:51 PM


Hey Thumpin 426: Thanks for the tip on the clockwise half turn on the timing screw. When your bike is at idle on the stand, will it cough and die if you whack the throttle open?

  • flyinguitars

Posted September 18, 2001 - 03:28 AM


Im having the same problem...when I wack the throttle at idle it dies. Bike is awesome other than that. I was going to go back and check the timing of the slide and see if it could be hitting when the throttle is really cranked. I dont remember if I was twisting the throttle as fast as possible when I originally did the mod. If that doesnt help I'll try changing the main...right now its stock. What main are you using?

  • Thumpin_426

Posted September 18, 2001 - 03:51 AM



As long as the idle is set around 2000 rpm it should'nt want to die as much however, mine tends to do that as well. I may be wrong but I think any type of engine would tend to bog or die if you mashed the throttle in less than 1/4 second especially if the idle is low. If you give it a quick double twist ( one little quick twist to get the rpm up then one full throttle) it should be alright. While riding I have no bog or hesitation anywhere. I can be putting along in any gear appropriate for the speed and mash it. BOOM, the front tire is off the ground!!!

I took mine out again on Sunday and tried playing with the carb settings a little more. I found myself right back to where I was on Saturday. ( My above first entry ) Everything is dialed in perfect!!! The only thing I may try different is going up from a 165 MJ to a 168MJ. I think my plug is a little too light.

Good Luck!! Later..........

  • mikeolichney

Posted September 18, 2001 - 06:13 AM


Thanks, Thumpin 426, I am injured right now and can't ride, but I was bummed when the mod didn't help on the stand. Hey, as long as it helps when moving. BTW, I am going to put in a 162 main ( I am at 5K ft) to make up for the lost fuel.

  • mikeolichney

Posted September 19, 2001 - 10:29 AM


OK, I went to the track and tried out the mod. My ankle is still lame so I couldn't do whoops or jumps. I went and rode wheelies and practiced starts. The BK mod is a noticeable improvement. It is easier to ride wheelies and my starts were better. But I can't say its astounding. My bike worked pretty well before. I still had a little bog on starts so I will go in on the timing screw for next time.

Its a good enough improvement that I am going to do the mod on my WR250F. That has a worse bog than my YZ426F ever had.

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