BK mod= AWESOME!

I know a a lot of you guys have tried this already, but I just got done with it. I did the 2003+ WR model on my 2004 WR450F. Here is what I did, which is what everyone does.

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What a simple yet God like mod :smashpc::thumbsup:. This takes your fuel squirt time frame from 1.7 seconds (I was running 2) down to 1/2 second by adjusting the AP. This is really just a quick tune mod, since the AP timing screw is already there, its just behind the throttle cover. The throttle response is incredible, there was no more bog coming off the bottom on the higher gears such as 4th and 5th and it reved quiker and got up to speed in no time. I did rejet, I put a 170 main and move my neddle clip down a peg. The bolt is a size 10 with a 24 thread pitch and it is hex head for quick tuneing. I also put silicone (not pictured) on the screw to keep it moving from where I want it and to keep any dirt out of the throttle cover. So there it is, a simple yet very effective mod.

Same thing happenes when you change the leak jet. That is what a leak jet is for.

Actually, that's what the button on the bottom of the diaphragm is for. The leak jet, while it can be used as a way of tuning other elements of pump activity, is intended as a way of preventing the pump from working during slow or partial throttle openings:

http://www.thumpertalk.com/forum/showthread.php?p=4415330#post4415330

http://www.thumpertalk.com/forum/showthread.php?p=6091344#post6091344

http://www.thumpertalk.com/forum/showthread.php?p=6091344#post6091344

The BK essentially does what the button does by limiting the stroke of the pump plunger.

Actually, that's what the button on the bottom of the diaphragm is for. The leak jet, while it can be used as a way of tuning other elements of pump activity, is intended as a way of preventing the pump from working during slow or partial throttle openings:

http://www.thumpertalk.com/forum/showthread.php?p=4415330#post4415330

http://www.thumpertalk.com/forum/showthread.php?p=6091344#post6091344

http://www.thumpertalk.com/forum/showthread.php?p=6091344#post6091344

The BK essentially does what the button does by limiting the stroke of the pump plunger.

The leak jet is constant.

The Diaphragm is constant.

The linkage is not; it has a spring on it to resist operation during partial or slow throttle openings.

Well, it worked, I have a 40 leak in right now, my whole carb has stock YZ 450 jets. And this just helped out a lot. I have no problems with power delivery, a few small ones are all gone.

The leak jet is constant.

The Diaphragm is constant.

The linkage is not; it has a spring on it to resist operation during partial or slow throttle openings.

That's incorrect. The purpose of the spring is three-fold:

First and most obviously, it transmits the driving force from the throttle segment of the pump linkage to the pump rod itself. The accelerator pump linkage is separated mechanically into two segments; the driving arm, which is linked directly to the throttle mechanism, and the pump rod. The spring is what connects the two segments, and with the spring removed, the throttle linkage would not move the pump diaphragm rod in any way.

Second, during a quick throttle opening, the spring limits the amount of force that is applied to the pump rod. Once a set amount of force is applied, the spring yields, and the pump diaphragm will only descend as fast as the spring force will push fuel out of the circuit.

Lastly, the spring allows the throttle to continue opening after the pump rod has bottomed out against the diaphragm button and the pump cover by allowing the driving segment of the linkage to wind the spring against the stalled pump shaft. If this were not done, the pump shaft would need to be allowed to move either through the entire stroke of the throttle opening, or only through the final portion of it.

The leak jet is constant, yes, but the flow in the discharge circuit is not. At slow or partial openings fuel escapes the discharge circuit via the leak. On faster openings, the leak is overwhelmed and fuel is driven up and out the pump nozzle.

The stroke of the pump is limited and controlled by selective button lengths on the diaphragm. This controls the total volume and the duration of the squirt.

Read the links I posted.

I have an 05 but I haven't even done the 0 ring mod and mine has never had any kind of a bog or hesitation, wonder why?

That's incorrect. The purpose of the spring is three-fold:

First and most obviously, it transmits the driving force from the throttle segment of the pump linkage to the pump rod itself. The accelerator pump linkage is separated mechanically into two segments; the driving arm, which is linked directly to the throttle mechanism, and the pump rod. The spring is what connects the two segments, and with the spring removed, the throttle linkage would not move the pump diaphragm rod in any way.

Second, during a quick throttle opening, the spring limits the amount of force that is applied to the pump rod. Once a set amount of force is applied, the spring yields, and the pump diaphragm will only descend as fast as the spring force will push fuel out of the circuit.

Lastly, the spring allows the throttle to continue opening after the pump rod has bottomed out against the diaphragm button and the pump cover by allowing the driving segment of the linkage to wind the spring against the stalled pump shaft. If this were not done, the pump shaft would need to be allowed to move either through the entire stroke of the throttle opening, or only through the final portion of it.

The leak jet is constant, yes, but the flow in the discharge circuit is not. At slow or partial openings fuel escapes the discharge circuit via the leak. On faster openings, the leak is overwhelmed and fuel is driven up and out the pump nozzle.

The stroke of the pump is limited and controlled by selective button lengths on the diaphragm. This controls the total volume and the duration of the squirt.

Read the links I posted.

Now you explained it correctly.

Well, gee, thanks for that, but there wasn't anything wrong with what I said in the first place.

I have an 05 but I haven't even done the 0 ring mod and mine has never had any kind of a bog or hesitation, wonder why?
Maybe you have your carb tuned correctly and you don't yank the throttle from idle to WOT and expect a response? Could be.

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