YZ400F carb: diaphragm rod length [PICS]

3 replies to this topic
  • Dudemeister

Posted March 23, 2010 - 05:12 AM


I have a question about the 99 YZ400F carb:

When the throttle valve goes up, the accellerator pump rod only goes down a little bit (so it only just compresses the diaphragm) before it bottoms out. I removed the diaphram: the AP rod now goes down pretty far, but I noticed the rod on the diaphragm itself (bottom side) is pretty long.

I suspect the rod to be too long (maybe someone put in the wrong one). Does someone know how long it should be?

p.s. I have no means yet to try to see if the carb squirts (but I cannot imagine it does since the AP rod barely goes down)

Edited by FrankV, March 23, 2010 - 07:35 AM.
pics added

  • Dudemeister

Posted March 23, 2010 - 07:29 AM


Here are some pics for illustration:

the carb:

Posted Image

throtte valve closed:

Posted Image

at wide open throttle:

Posted Image

the diaphragm:

Posted Image

If I see this, I'd also think the AP rod is coming out very far, but I don't think this can be tuned? (apart from shortening the rod itself)


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  • grayracer513

Posted March 23, 2010 - 09:12 AM


The button on the diaphragm goes down, and is available in different lengths. The length of the button is used to adjust the total length of pump diaphragm travel, which does not need to be very long. Once the button bottoms out, the rod stops, forcing the AP linkage spring to wind up in order to allow the throttle to open farther.

Here's some good info on the setup of the 400 AP:

http://docta-zaius.f.../whats_new.html (watch for pop-ups)

  • Dudemeister

Posted March 23, 2010 - 02:06 PM


Great info, I'm going to look into that. Thanks!

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