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Discussion Starter · #1 · (Edited)
So I finally got to start on my Winter maintenance projects.
First on the list was a carb rebuild.
The bike had been setting for at least a year this time around when I got it.
Living at 4,000 ft elevation it ran pretty fair with a little bit of choke applied.
I still understood there were some issues with it since I could see staining on the bowl gaskets and a few other places.

Noticing that the pilot screws were visible I assumed someone had attempted a rebuild or major adjustment previously.

When I pulled the carb rack, I noticed a few of the hose clamps were missing and a couple of other things like 2 plenum cover screws were missing.

I had purchased a Randakks kit earlier so I was finally getting to it

As I pulled the Air Cutoff Valve cover I found this really nifty hole/tear in the diaphragm that was about 3/8 long by 3/16 inch wide.

It would seem that this would allow vacuum from number 4 cylinder to pull air for the Slow air supplies on both banks. Essentially making #4 lean but also influencing the other cylinders.

As I was tearing down the carbs I checked pilot screw adjustments. They ranged from 2 3/4 turns to 1 1/2 turns on #4.

As an aside, the PO of the mechanic sure got happy with the stickem when putting in the gaskets. The gaskets were completely flat and I noticed several places inside the bowls where the gasket cement had seeped/oozed into the bowls.

Anyway, the rebuild was much needed due to fuel crap in there and also some plugging in the jets.

So back to my basic question, What does the Air Cutoff valve do?
 

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It cuts the air to the slow jets when you get off the throttle. Makes the engine run rich temporarily.
 

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1993 gl1500, 1976 gl1000
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And the reason for it is to prevent backfiring during decel.
 

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Discussion Starter · #5 ·
Sorry it is a 1984 1200 Interstate.

Thought I had forum setup to post my sig.
 

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My bike used to backfire upon heavy deceleration every time until I replaced the air cut off valve. I tried all of the usual and easier fixes first but with no change until that valve was replaced.
 

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The cover on air cutoff valve was warped on my bike 10 minutes with a file fixed the cover.
 

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Discussion Starter · #8 ·
I did some further digging as I cleaned up the carbs and flushed the paths in the carbs in this rebuild.
It appears the Air Cutoff Valve is activated on Decel/closed throttle from the vacuum connection in the #4 intake. High Vacuum open the valve allowing plenum air to to pass into the Vacuum slide chambers.

This essentially Ambient pressure air would decrease the vacuum level that lifts the slides and that would then close the slides.

Closing the slides would of course shut down the engine and fuel flow to that of an idle.

A torn valve would prevent most of this from happening since the valve is under spring pressure lacking vacuum.
 
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