Wr400 AP timing screw

Hey guys,

Just bought a 2000 Wr400 and the bog off idle is ridiculous. I did the BK mod, so the duration of the AP squirt Is about 1/2 a second, and the jetting seems perfect. Can anyone tell me how many turns out their Ap timing screw is so that I can use that as a baseline? Also, should I change my leak jet, or is it fine after doing the BK mod?

It's not about turns, it's about gap.


Do you have the slant carb or the CR carb?


Your factory manual has an AP timing gap setting procedure


You can also do a visual to make sure it just barely misses the slide, everytime. 

For a precision measurement of that, you really need to use a video camera.


Squirt distance is a function of the AP diaphragm type (short/ medium / long rivet. You want long for a 426)


Squirt duration is a function of the leak jet size


You really want closer to 1 second, especially for on/off/on throttle situations. That would mean a smaller leak jet (50/55)

If you can change the duration of the AP squirt, then what is the purpose of the BK mod?

The only people who do the BK mod are people who don't understand how to tune the apump system with the leak jet, upgraded return spring, and diapgram options.

Ah, I guess that applies to most people (and me previously) then.

Sorry, no intentions of being dickish, because the mod is effective for those bike that come with no leak jet and  the short rivet, but is unable to 'take' it. 

Turns out my carb doesn't have a leak jet. However, I found that the last goon to work on the carb installed the rubber housing/ connector that goes into the air box boot without a gasket where it meets the carb, so there is a large air leak. Could this air leak also contribute to the bog?

Do you mean the air boot? It has no gasket on either end....


You still have not stated weather the pump is squirting, for how far and for how long.

Its not the air boot, its the piece in this picture.

ImageUploadedByThumper Talk1435498767.392043.jpg

There is supposed to be a gasket between this black rubber piece and the carb, but there isnt, so there is a gap between the carb and this piece.

The AP squirts about 6 inches for 3/4 of a second.

That is the air boot maniford.

It needs a gasket.

It will cause a hanging idle / lean condition and a massive loss of vacuum pressure over the needle jet.

Might cause bog, I've never seen it missing before.


Your pump should be squirting 6 FEET for about 1 second


If you have no leak jet, than that is not the issue, as you are running what would be essential the largest leak jet, and still having bog.  Pleanty of fuel.


So the 'pump' is not pumping hard and deep enough.


You need to look at the diapraghm to see if it has the long, medium, or short rivet.  You want the short rivet (longest 'pump' distance).

Edited by KRANNIE

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