BK – Where’d you go?

BK – Where’d you go?

Your little adjustable accelerator pump-stop mod is making quite an impression over on the WR side (that’s where all the jetting geeks hang out). You didn’t answer my email (no worries bud) but I figured out exactly what you were referring to as soon as I looked at my carb.

I’ve got a replacement spring and screw for the pump timing adjuster on order, hopefully it is long enough, but the spring I’ll be able to use regardless (this part is also a 4mm screw).

It will be a few weeks minimum, but I’ll post the results (and hopefully some pic.s) when I’m done.

BTW I’m also going to try an FCR Air Screw in place of the fixed Pilot Air Jet. This is probably a better mod for those of you running larger pilots but I think it is under ten bucks.

Here’s another post on the WR side about the air screw with phone numbers and PNs.

[This message has been edited by Hick (edited 02-22-2001).]

Yo Hick

I just wanted to let you know I installed the

accelerator pump stop on my 2000 yz426. I used a 20mm long socket head screw it seems

about right. The hard part is the .3 second

spray duration its to fast to time.

Anyway it worked out real good, almost looks

stock. Haven't got a chance to test it for

real yet. Plan on doing that Sunday. The

good thing is you can adjust it even with the

dust cover on and if you don't like it, just

remove it. Have fun



Originally posted by MM:

The hard part is the .3 second spray duration its too fast to time.

Thanks for the tip bud.

As for the duration, I believe BK stated the gap between cam and screw after adjustment would be 2 to 3 mm, MM.

Also, Clark Mason, a real knowledgeable tuner on the WR side, found that at a pump stroke (via a Fact. R&D pump cover) of 1.78 mm worked best.

So timing the duration isn’t the only method of measurement, you can ball-park the gap, although we don’t know at what additional timing, or you can measure the stroke of the pump.

I believe Clark has his WR more or less set up like a YZ (jetting, cam timing & exhaust), which is probably why R&D found it troubling to tune, so his 1.78 mm stroke should be a good starting point.

As for the timing, if you have altered it, James Dean (the other half of the WR dynamic jetting duo) found that the stock setting was one turn beyond zero delay at idle. Or, in English, w/ slide resting on idle stop, turn the timing screw out until the actuating rod makes contact with the pump diaphragm (you can tell either by feel or by removing the pump cover and looking for the rod to be flush with the bottom of the carb body), then go one full turn in.

Hence the one turn of delay at idle.


Thanks for the info. For starters I set the pump timing so it just clears the slide and

the duration a quick squirt ".3" sec. this left the stop gap at .024 of an inch checking it with a feeler gauge. At 2mm to 3mm the squirt stayed on over a second. Just riding

it down the street in my hood it seems ok. I will find out for real on Sunday. And also

see if I can measure the stroke.



Thanks for the update. I’m still waiting on a screw and spring.

If you want an exact pump stroke measurement remove the pump cover and measure the length of actuator rod that extends below the (now exposed) bottom of the bowl with the actuator arm bottomed on your pump stop. Since the pump will start operation as soon as the rod extends below the bowl, depressing the diaphragm, the distance it extends downward will be the pump stroke.

I can’t wait to try this on my bike.

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