Probably A Silly Newbie Question....BK/GB Mod?? Whas Dat Yo?

11 replies to this topic
  • JYD

Posted June 14, 2004 - 04:00 PM


anyone care to explain?

  • John_Lorenz

Posted June 15, 2004 - 03:43 AM


Here ya go

GBmod Explained

  • Fastest1

Posted June 15, 2004 - 03:47 AM


It is a modification (drilling a hole and tapping it to accept a screw) to make the accelerator pump squirt adjustable. A lot of these bikes came with too long of a squirt and there was no way to adjust them. It was originally given to Garrett Berg (a beloved TT member who is no longer with us) by Brian Kinney (Tim Ferry's mechanic at the time) hence the BK/GB mod. BTW the too long of a pump squirt caused bogging and plug fouling. :thumbsup:

  • DrThumper

Posted June 15, 2004 - 05:15 AM


Don't waste your time and effort with the BK Mod, first of all its almost impossible for the shade tree mechanic to time the squirt. Second, you can get the same performance if you lean out your pilot jet and install a Free Power Now, and, if you want to go one farther, install a P-38 Lightning. No bog, no hassles, better low end.

No guess work, no modifying parts, no chance of fu@*ing up your carb. No chance of the modification cuasing other problems.

  • JYD

Posted June 15, 2004 - 05:55 AM


what is a p-38 lightning?

  • John_Lorenz

Posted June 15, 2004 - 05:58 AM


The Far more expensive version of the 10 dollor GB mod that works great. :thumbsup:

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • Fastest1

Posted June 15, 2004 - 10:56 AM


The BK/GB mod should cost about 1.50 at the most. To those who doubt it, you never did it. Leaning out the pilot is not the cure nor is the power now. I run a power now and it was a good improvement. The leaning of the pilot does nothing regarding the pump squirt. Yeah maybe you cant time it so you know the duration in milliseconds but you can improve the response by turning a screw and see what happens, if you dont like it, remove the screw and you are back to stock. The p-38 cost how much? 70.00 bucks?

  • John_Lorenz

Posted June 15, 2004 - 11:22 AM


10 bucks if you dont have a drill bit and tap :thumbsup:

But John is correct, the GBmod is a great mod and works.

  • JYD

Posted June 15, 2004 - 02:30 PM


ok so, no one has explained how to, or what the mod exactly is.

  • Satch0922

Posted June 15, 2004 - 03:03 PM


Which mod? John gave you the link to the BK/GB information. That is really all you need. PM one of the guys who did the P-38 and get their info if your so inclined to go that route.

  • John_Lorenz

Posted June 16, 2004 - 03:37 AM


Which mod? John gave you the link to the BK/GB information. That is really all you need. PM one of the guys who did the P-38 and get their info if your so inclined to go that route.

I guess you missed the link or failed to read it so here it is.

In Essence it shortens the Pump duration and lessons the amount of fuel injected at idle to 1/4 throttle by allowing a adjustment to fine tune the spray and duraction.


First I wish to say that this mod is entirely due to Brian Kinney, Garrett Berg is the first to have detailed and posted the mod to the Thumpertalk forum.

I am merely trying to explain the details on proper setup after the mod. Thanks Brian Kinney and Thanks Garrett Motoman393 Berg. Also note, the pics are from Garrett's site as well. I enhanced them for the settings detail.


Taken from Garrett's Web Site

This accel pump mod is ONLY for the 00'-02' YZ426, 01'-02' YZ/WR250F and 00'-02' WR400/426) this WILL NOT work on 98-99 YZ400's and 99 WR400's. When this mod is done correctly is will make your bike run and start better. It also will give quicker throttle response and eliminate the bog off the bottom. Your jetting will most likely need to be richened up since this mod shortens the duration of gas sprayed (from 1.7 secs stock to .3 secs)

The Procedure that Brian Kinney gave me:

Brian Kinney is Tim Ferry's Factory mechanic (and the originator of the BK mod) and these were his instructions...To limit the accel pump spray length you must drill and tap the pump cam stop and install a 4mm screw/spring combo. That will contact the pump cam. A hex socket bolt works well approx. 25mm long. Remove the subframe so you can look down the throat of the carb and with a stopwatch time the length of the pump spray when you stab the throttle. I usually click the stopwatch at exactly the same time I turn the throttle and click it again when I see it stop. It may seem weird but it works! Also you need to adjust the pump timing screw so that it does not hit the slide when it is raising. The timing is close when the spray just misses the slide. Then set the duration of the spray to .3 seconds with the adjustment screw you just installed. This may sound complicated but is the only way to get rid of the pesky bog off the bottom.
Helpful Tips from me:

You don't have to use a 4mm bolt I used a 6-32 1" long flat head screwdriver bolt. I also used a spring out of a pen that you write with. My stock pump timing was around 4.5 secs and now it is around .35 secs (maybe that was why is flooded easy and was sometimes hard to start when hot?) Also when drilling put a piece of metal (like a hacksaw blade) in-between where the drill bit will poke through and the black pump thingamajig LOL. The squirt of the gas was also off on my should spray gas so that it just clears the slide...but mine sprayed when the slide was about halfway up the stroke!

Just by starting up the bike I could literally "feel" the difference, it felt like the snap of a 250 2 stroke and didn't have any bog whatsoever! You will really be amazed by what $4 does to the performance of this bike! The jetting I ended up with: 168 main, 42 pilot (1 3/4 turns out), needle in middle position.

The below image shows the Mods main component. This is the duration screw added to lesson the duration of the fuel Squirt
Posted Image

The Second image Describes the GB/BKmod in conjunction with the Accelerator timing screw. More of both these will be explained below.
Posted Image

The 3rd Image is the side view of the complete mod and how it will work.
Posted Image

The last 4th Image is a side shot looking somewhat down the back side of the carb. I will now explain the setup.
Posted Image


So to begin, if you have not done the mod yet follow the mod completely as Motoman's instructions specify in the first part of this post above.
One side Note Very important. When drilling the 4mm hole into the Carb, place a thin piece of metal (Hack Saw blade) between the exit and the plastic cam.

In pic 1 I used a 4mm hex bolt, much easier to adjust.
Just set the bolt into place at first at the edge of the hole or just leave it out until the timing is adjusted.

Now for the timing
In pic 2 it shows the pump screw, it is important only to adjust this clock or counter clock wise in 1/8 turn increments. It is very sensitive and if you pump is working correctly you will see the spray change effectively.
Note Make sure the float bowl has Gas in it for this part, and don't do this near a flame

Take a look at pic 4, but you should be looking down the back side of the carb, keeping the carb as level as possible.

Turn the cam on the carb, you will notice a little brass nipple in the right corner just in front of the slide. This is the squirt nozzle.

If you twist the cam you will see a stream of fuel hit the slide as it opens.

so with that in mind if you turn the Cam and you see the squirt immediately hit the slide and the slide is not moving, you need to turn counter clock wise the Pump Time Screw 1/8th turn.

You should see an immediate effect on when the squirt starts.

Remember the squirt must start just as the Slide begins to open

Once you have the time done, it may take a few sets to get it correct. You can now work on duration. As you noticed the duration is long and the squirt has plenty of fuel.

So now comes the duration
I don't understand how the .3 or .5 duration stated, I am not that fast to hold a stop watch in one hand eyeball the spray and turn the cam at the same time. I am sorry but logic tells me unless you are a Cyborg, its impossible.

Anyway the way I did this is to visually adjust the duration by sight and feel. The 426 only needs enough gas to get rid of the bog, on the other hand not enough and you still have the bog (Fuel Starvation).

The Distance between in Pic 2 on my sled is about 1/8 of an inch. I set this by watching the squirt as I opened the cam up. I tried a couple of diff settings but found that all you need is a split second of fuel to get the beast to breath fire at bottom.

So that is it except that you may have to adjust it for you area of ride, altitude, humidity, and weather will effect the GB/BKmod. It does take some time but to adjust and get correct is a big payoff on the 426.

I used the Hex Bolt because once I was able to get the Mod complete and the bike back together a simple quick adjust to fine tune in the woods was perfect.

Again, it is a great mod, it can have dramatic effect on your ride if done correctly.

Hope it helps

  • JYD

Posted June 16, 2004 - 03:45 AM


thanks, what i needed were the pictures, since they did not come up on the link.

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