YZ426 Carb Adjustment?

Joined
Mar 1, 2001
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#1
I mistakenly adjusted the "Pumper adjustment screw" on my 2000 426 Keihin FCR carb.(the screw is located under the cable pulley on the right side of the carb and adjusts the amount of gas that is squirted everytime you twist the throttle.
Yamaha recomends that you don't mess with the stock setting (MY MISTAKE). I can't find any reference in the Shop Manual on how to adjust this. ANY HELP WILL BE GREATLY APPRECIATED! Sincerely,
LopeMX

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Joined
Dec 20, 2000
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#2
I see a certain lack of responses here, try posting you question on Thumpertalk.com. These guys have tons of knowledge with the 400/426.

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Ahh, the sound of a 426 in the woods!
 
Joined
Aug 15, 2000
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#3
Oops!

First off, that screw just adjusts when the pump begins delivering fuel w/ respect to throttle position and doesn’t really affect the amount of fuel delivered.

I think most bikes are about 1 turn in from zero play at idle, or one turn of delay. This will, of course, vary depending on your idle setting (initial amount of slide opening).

To set this then you would need to find the point at which the actuating rod is just beginning to move the diaphragm. You may be able to feel this at the lever, or you may decide to remove the pump cover. Then with throttle off (slide resting on idle setting) turn the screw out until the actuating rod begins to protrude below the carb body, then turn one full turn in.

If you have too much delay (screw is too far in) the pump won’t start soon enough and the motor will bog, too little and the pump will spray fuel against the back of the slide which isn’t an ideal situation either. To verify this you can observe the pump nozzle which sits just behind the slide with the carb on the bike and the subframe and airbox removed.

You may try the latter method and just eyeball your pump timing by setting it to begin spraying just as the stream clears the slide. Then you can fine tune the timing to get the best throttle response. FWIW, on my ’00 YZF I liked it better with ¼ turn additional delay over the factory setting. In short, leave it wherever you and your bike like it best.


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'00 YZF