Air Cut Valve

The WRF has an air cut valve circuit (ACV), also known as a coast enricher, on the left side of the carburetor (Figure 1). The YZF does not have an ACV. The Air Cut Valve (ACV) lets extra air flow to the pilot jet during most running conditions, but not under deceleration. The carburetor has 2 air passages to the pilot jet and one (the ACV passage) closes of during deceleration. This richens the pilot circuit on deceleration to reduce popping. The Air Cut Valve "cuts" the air passage off by using vacuum to pull opposite the spring and diaphragm. When you back off the gas at high revs, the ACV richens the air/fuel mix to stop the bike from popping and banging through the exhaust. In the perfect world, the air cut valve would ONLY richen the circuit on high RPM deceleration (to prevent backfire).

How It Works:

The center hole with white plastic valve leads directly to the pilot jet air circuit, along with the pilot air jet. The diaphragm spring and pin normally hold the valve open.
The hole on the left leads straight into the intake manifold for vacuum. This pulls the diaphragm pin away from the center hole on deceleration, closing it.
The hole on the right leads to the airbox side of the slide. It allows atmospheric pressure air to flow thru the passage in the center when the vacuum is lower, and leans the pilot circuit. The WRF's have a bigger pilot jet(42-45) than the YZ's(40-42) to compensate for the added air.
The ACV simply cuts the air to the pilot jet in half when using engine braking.

Unfortunately, the spring tension against the vacuum at mid throttle position also richens the circuit and keeps changing. Some riders have suggested that it is hard to set the fuel screw by ear because the ACV constantly changes the mix. Others have indicated that reading the plug color and jetting the pilot circuit are also more difficult due to the ACV. Especially if you have YZF timing on your WRF, it's a good idea to modify the ACV, so jetting will remain more consistent.




In searching through the TT archives I came up with several different ways riders have inactivated this circuit. I do not know which one is the best option. If you disable the ACV with the center hole closed, the pilot circuit will run richer both at idle and on deceleration. If the center hole remains open, the pilot circuit will be leaner under the same conditions.The method I used is detailed in the Procedure section at the end.

Method 1:

To simulate the YZF exactly you have to change the PAJ to 100 and close the circuit (remove the spring pushing in diaphragm). As such, the PAJ change leans the bike and the closed circuit richens it.

Method 2:

If you want to stay with the 75 PAJ (richer than the YZF) you have to open the circuit by leaving in the spring and diaphragm and plugging the vacuum port.

Method 3:

Remove the spring and diaphragm and make a piece of rubber to plug up the “o-ring” hole. This method seems to be the most common.

Method 4:

Remove the spring and diaphragm and make a larger rubber plug to plug off the whole circuit (both holes).

Method 5:

Removing the spring, leave the diaphragm and plug the “o-ring” hole.

Method 6:

Plug the “o-ring” hole (toward the engine), cut the pin off the diaphragm, remove the spring and put diaphragm back in.

Method 7:

Take spring out then put washer behind valve.

Method 8:

Plug the “o-ring” hole, leave the spring in and put the diaphragm back in backwards.



  1. Remove ACV cover and carefully remove the spring, the diaphragm.

  2. Using a leather punch cut a small plug from an old inner tube that fits snug in the whole in the small O-ring.

  3. Cut a small round piece of material (such as an inner tube) that will cover the cavity with the door and the small port in the top.

  4. Replace the diaphragm in the ACV cavity backwards with the pin pointing out.

  5. Replace the spring and the cover.


 Update for 2003 Model

I have seen no specific information regarding this mod for the 2003 models. In looking at pictures of the 03 carburetor, the ACV appears to be unchanged. Suggesting the mod is still applicable.


 Update for 2004 Model

I have seen no specific information regarding this mod for the 2004 models. In looking at pictures of the 03 carburetor, the ACV appears to be unchanged. Suggesting the mod is still applicable.


 ThumperTalk Archives (updated 09/02/03)

ThumperTalk ACV plug method

ThumperTalk air cut valve

ThumperTalk Air Cut Valve and Neutral Wire

ThumperTalk air cut valve mod

ThumperTalk ok gurus, answer this question

ThumperTalk what the hell is an air cut valve

ThumperTalk Woody426's awesome discovery! ACV.....Proper way to close it off WR450 Stutter Gone!

Listed here are what I consider the most helpful threads. There are more threads on this topic in the ThumperTalk archives. To find them, search the YZ250F/WR250F forum. Suggested keywords: ACV, air cut valve, coast enricher, octopuss.



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Last Updated 07/16/2004