The YFZ has an air cut valve circuit (ACV), also known as a coast enricher, on the left side of the carburetor. 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 YFZ'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 YFZ, it's a good idea to modify the ACV, so jetting will remain more consistent.

 

 

 

Note: this is a YZF not a YFZ but you get the idea!

 

 
To Block of this there are several methods. I simply filled the hole with a rubber compound.

here is a slightly alternant method but it still does the same thing

  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.