Air Cutoff Valve Overhaul

You might consider this item: Randakk’s GL1000 Carb Rebuild Video

The air cutoff valve is a simple but vital component of the carburetor system on GL1000s. Its function is to eliminate backfiring on hard deceleration (throttle fully closed). Hard deceleration creates a high vacuum signal in the intake tract which activates the valve. When activated, it restricts the flow of air to the external idle circuit air jets thereby richening the mixture. This slightly richer mixture is calibrated to eliminate backfiring in this operational mode.

Here’s a tuning tip. If you haven’t replaced or overhauled your air cutoff valve in a while, I recommend you do so as a precautionary measure. I recommend an interval of 25,000 miles or 3 years. These can fail catastrophically or slightly. When they fail completely, you’ll have very apparent backfiring on deceleration. Often, they fail in a more insidious way by shifting the “function point” of the valve to a higher vacuum / rpm level than designed. So, even though the diaphragm is intact and will hold a test vacuum…you can get an excessively lean condition resulting in poor off-idle throttle response. Typically, this may also lead to a condition where the bike requires lots of chokes until it is completely warm before it responds well to the throttle.

In lieu of the expensive Honda item, aftermarket air cutoff valves are available from a variety of outlets at reasonable prices.

The actual rebuild procedure is straightforward and well covered in the Official Honda, Clymer and Haynes Manuals. Your carbs need NOT be removed to rebuild the air cutoff valve.

1. Remove the valve which is held to the plenum by the 2 screws at the base (not the 2 screws on top which secure the cap).
2. Disassemble the valve by removing the top plate held by the 2 short screws.
3. Underneath – you will replace the rubber diaphragm and spring.
4. When reassembling, the spring goes on top of the diaphragm (installed position) inside the tower portion of the cover.
5. The diaphragm mounts with the actuating pin pointing down (installed position). Be extra careful here – the diaphragm can be installed upside-down rendering the air cutoff valve useless in operation!
6. Don’t forget to install the small O-ring that seals the cap to the body of the air cutoff valve
7. Use 2 new o-rings to mount the rebuilt air cutoff valve to the plenum.

Note: Randakk’s GL1000 Master Carb Overhaul Kit includes an air cutoff valve overhaul kit…details here. If you use the air cutoff valve rebuild kit I supply, depending on my supplier – there might be an extra small seal you won’t use in this application. Discard this extra seal.

While you’re at it, you might as well replace the vacuum hose to the air cutoff valve. This hose is 3.5 mm id (good substitute is 5/32″ id).

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  1. I have some idle problems with my gl1000 75 and i think it has to do with the air cutoff valve, i think that the rebuild kit i bought for this air cutoff valve is not with the correct spring, can you tell me the dimmensions of the spring acting on this valve?, and can you give me some information about the forces needed to compress this spring?.


  2. Tom Ireland says:

    I have replaced the small O ring that seals the cap to the body of the valve (the original ring seemed more like a hard, flat rubber washer). The rubber diaphragm seemed fine. Vacuum testing once reassembled reveals that it cannot hold a vacuum completely; it does lose the vacuum very slowly. At what point is this a problem?

  3. David Lytle says:

    On my 1977 Gl1000, I was able to get access to me carb caps (to do the mid range fix), without removing the air cutoff valve (or taking the carbs completly off). I just disconnected the hose. However, the base of my valve is held in place with two 2.5 mm hex head bolts, not screws. I don’t know if these are factory installed. I cannot remove them, as the heads strip before I can back them out. Any thoughts on doing a cutoff valve overhaul in situ–with the base attached to the plenum?
    David L., Portland, OR


  1. […] 22. Failure to rebuild the air cutoff valve. Closely related: Failure to rebuild the air cutoff valve correctly. The diaphragm can be installed upside down! More details here. […]

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