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Accelerator Pump Adjusting Screw

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I'm in the process of fine tuning what

was a really piss-poor setup on a 99

WR400 I bought.

It runs pretty good now, but when I

give it a good throttle blip off idle

in neutral, the bike gasps for air and

sometimes dies.

I'm looking at the accel pump and it

appears to have next to no stroke (less

than 1 mm?). The archives have thoroughly

confused me.

Let me ask:

The linkage pumps when the rod is

pushed down, correct?

Where is this fabled accelerator pump

adjusting screw? I only see the idle

screw that is connected to the big

thumb knob.

Are you talking about the screw on

top of the slide that can only be seen

when you remove the valve lever housing

cover? (The one mentioned in the tech

bulletin "what to do if your WR400 won't

idle at any setting")?


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On the 99 and 98 WR and YZ's there is no adjusting screw on the accelerator pump. the only adjustment on the pump setup stock is the pump starting position. It is controled by the little "FORKED" stop behind the black plastie pump rod holder. Spread it or closing it affects when the pump mechanism starts pushing on the top of the pump rod.

To stop the pump stroke sooner you need to stop the diaphram downward motion sooner. You can do this by purchasing a P-38 lighting pump bowl body which incorporates a taller center section stopping the pump stroke sooner.

Stock the stroke is approx 3mm (0.120")Mine is currently set at 1.33mm. By shortening the stroke it significanly drops the amount of fuel shot into the carb throat during each activation.

To accuratly measure the stroke on 98 and 99 take the complete FLOAT bowl off the bottom of the carburetor and take off the activating rod. Insert the rod into the hole in the top of the pump bowl and mark (scribe with a razor blade) the side of the rod when it just touches the top of the diaphram. Now push it in and mark the rod again. Get a set of calipers and measure between the two marks. this will tell you the maximum possible pump stroke.

If you were real creative you could drill through the bottom of the pump bowl chamber thread the hole and use as lock nut on a screw along with a sealing washer to seal in the gas and then have a adjustable stop. This is a little much for me as I would not trust the seal!

I went for geting a P-38 after market pumb chamber bowl with a limited stroke (0.050") initally and then modified it by slowly increasing the stroke to where I'm at now, testing between mods. this was a very long process.

On 00 and later carbs it sound like a external stop can be added to adjust the pump stroke stop location.

Good Luck


[This message has been edited by Clark Mason (edited 03-11-2001).]

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