98 wr accelerator pump blues

I just bought the bike and I noticed it was hard to start cold... I started checking some things and I have found that the Accelerator pump is not working... When the bike is warm it runs great and starts easy...

If I look down the throat of the carb when I twist the throttle... no gas...

Examining the linkage the pump rod is not moving... I cant see anything that looks like it is broken... When I roll the throttle there is a plastic piece that is attached to the pump rod and it does not move... the metal tab next to it (for lack of a better term) moves but the plastic piece attached to the rod stays stationary... I thought about prying on the rod to see if it would move but then I thought that may not be a good idea...

For all I know the guy that had it before me had it apart and put it together wrong.

I new to this setup. My first big thumper... Any suggestions...

I'm going to the dealer tomorrow to look at a new bike to examine the way it is set up so I can see if something is hooked up wrong or broken...

Dont bother looking at a new WR, they have a smog device that is in the way pretty good and you wont be able to see much of that side of the carb.

Do a search for "BK Mod" and you will see that back in the day we were trying to minimize the accelerator pump squirt duration to help with throttle response off idle and when done properly it works great.

The guy that had your bike may have not known about the BK mod and killed the accel pump completely on purpose.

Did you get a manual with the bike?

Clymer makes a pretty good one that will help you a lot to understand what is going on with the whole bike and is a good buy.

HERE IS THE LINK For Garrett Berg's (MotoMan393) web page that has a lot of great info on how to make these bikes work a lot better.

Dig around in there fro a bit and see if any of his info is helpfull to you.

The manual I have is not listing allot of details... Factory manual Goverment diagrams ans exploded views... nothing detailed about the accelerator pump operation.

Thanks for the link... I just looked and unfortunatly it says this mode does not work on my bike... Looks to be a different carb.

I do appreciate your input...

Any other suggestions...? I kicked this beast for 20 minutes yesterday... nothing...

I have spark and compression... Ran fine last weekend... A bitch to start...

I'm going to slide the boot over and squirt some fuel in the throat this afternoon...

Take the carb out and disassemble the accelerator pump assembly... They have a tendancy for getting crap on the rod and freezing up. Remove the diaphragm from the housing on the bottom of the carb and also clean that up, my guess is that you will find a little rust not the top of the diaphragm. If you need a new diaphragm, you can order a new one from Yamaha (stock) or get one from JDjetting.com. The JD one is a little thicker in material and will help with any bogging you may have. Also, the JD Jetting kit is a great addition.


I read up on the BK mod in regards to my 99 WR400 yesterday. It doesn't apply to the older WR's. Only 00 and up 400's, 426's.

Thanks for the tip CuLayTur

I read up on the BK mod in regards to my 99 WR400 yesterday. It doesn't apply to the older WR's. Only 00 and up 400's, 426's.

The accelerator pump mod for the 99's involved a wheel collar like you would find on radio controlled cars. You position the collar on the rod to adjust the squirt duration.

Hi Racejmac, want you want to look for is info that is mostly referred to as the "Taffy Mod" so try a search under that. For the 98 & 99 YZF & WRF FCR1 carbs you have a external rod that pushes down on the AP that I think you said does not move. I believe I have read that this rod can get corroded and freeze up. You may need to clean it or replace it to make it work right. I am not sure on this but you can ask this forum that question and I am sure you will get some answers. Second, their is some pictures that I could not find but have made copies of that I can dig up that shows how to reduce the AP 4-5 second squirt duration down to around 1/3 to 1/2 second duration because it squirts to much initial fuel temporarily flooding your engine causing the bog. The fix is going to the hobby store and getting what they call a RC collar. This collar is just 1/4 " round X 1/8" thick (about) piece of medal that you will want to get with a 3/32 " hole. This collar will have a set screw going threw it that will tighten up against whatever you put threw the center hole. What you want to do is slide that collar onto that AP rod and slide it down to where it will reduce the travel that this arm can push down against the AP thus reducing the duration of the squirt.

Hope this helps. If you want I as I said, I could dig up my filed info and even send you by mail with one of my left over collars since I had to buy them in a pack of 4.

Good Luck with your fix


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