BK Recommended Carb Mod (No Hiccup allowed)

In a couple of past threads BK indicated he was (possibly) going to put up some info, possibly including pics, on the set-screw addition to the carb to limit accellerator pump spray, thereby eliminating the hiccup.

Other than the general description he gave, does anyone know if more information was made available? I sent last month to BK direct an e-mail, but never received a response....... Nationals can be a busy proposition.

I just want my (still new / unridable) 2000 to run just like the marketing wherewithalls at Yamaha intended.

Oh, how I long for a bike that rips rutts in dark loamy soil......




This is an easy mod and works really well--plus it helps starting after tipping over.

Although I don't believe any pictures were posted, you will have no trouble visualizing what to do. Just remove your carb and look at the accelerator pump area (RH side, under plastic throttle cable cover) as you work that throttle open a few times. Notice the direction the plastic accelerator pump cam is rotating (CCW) and visualize where a limit screw would have to go to restrict the cam movement. Once you have done this you will notice that the carb already has a halfmoon-shaped boss cast into the housing to accept the screw!! All you have to do is drill/tap a hole in the boss. Then set it up as BK describes in his post. You don't even have to disassemble the carb, so it doesn't take 30 minutes to do. (Tip: While drilling the hole, place a hacksaw blade--or steel straightedge--underneath where the bit will poke through, this will prevent it from accidently gouging the plastic accel pump cam.)

Its easy. Go for it.

O.K. VN, I'm going to give it a try.

Thanks for answering.


I have done this also, it was very easy, like vxnx1w said as soon as you look at the right side of your carb you will see what we are talking about.

BUT, I don’t think this will work on the slant FCR (pre ’00).

I used a 5 mm screw, I think about 20 mm long and used the same spring that is used on the pump timing set screw (5JG-14531-00-00).

I have my pump set at about 1 mm of pump stroke, you can set it with carb off the bike by measuring the distance of travel at the rod. With a narrow screwdriver I can (just barely) adjust mine w/ carb still on bike, the shock is in the way but it is doable.

A mod I very, very highly recommend to anyone of modest mechanical ability. If you can jet your bike you should do this, then try jetting the bike again w/ the pump turned off.

Here is an old thread on the topic with more detail.


I made the modification too. It was easy enough if you have the drill and tap. The screw on mine was 4 X .7mm . The spring is the same as the pilot air screw spring from Sudco - P/N 021-235. The screw length with this was 16mm long. The screw gets set at about 1 1/4 turns from "touch point" according to BK. When you set this screw, be sure to take into account the extra gap due to the delay to the pump start. This delay is set with the other screw which is already there under the black cover.

There were discussions on the 250F forum about this. After making the change, I have consistently found that the bike will run better at 1 clip richer than before.

Do a search on "BK Foseca" in the YZ250F forum to see what SUnruh found via email from BK.


[This message has been edited by James Dean (edited 05-21-2001).]

My bad, I used a 4 mm x .70 screw as well.

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