Found My Vacuum Leak, Finally
dan7uk:
I purchased a 99 Interstate about 4 months ago and notice when letting off the throttle bike would not return to idle immediately when shifting. First adjusted the throttle play, not the problem. Asked the board here what I might check next and everyone responded that it sounds like a vacuum leak. Next thing I did was check all the boots and I replaced all manifold O rings, not the problem. Next thing I did was I desmog the bike hoping I would see where the leak was, didn't see anything that I saw would indicate a problem, strike three.
It was a more of a nuisance problem so for the past four months I lived with it. Out of desperation I purchased a Digi Sync gauge to check the sync on the carbs. All the carbs were dead on except #5, almost no reading on gauge no matter how much I adjusted the sync screw. Bingo problem found. Used a dental mirror and gave #5 a closed inspection. Found a small hair line crack in the hose on the Air Cut Valve. I replaced the hose and put the gauge back on it #5 was now reading.
The Digi Sync save me, it narrowed the problem down to #5 carb and gave me a place to concentrate
on. I was lost until then. So don't forget the hose on each carb off the Air Cut Valve, it drove me nuts.
This is not a commercial for Digi Sync any Vacuum gauge could of found it.
Part #24 is the valve & 28 is the hose.
https://www.ronayers.com/oemparts/a/hon/5053ea2ef870021c54be3e8d/carburetor-component-parts
HayHauler:
Oh, THAT hose. I am glad you included the ronayers.com diagram with the part numbers. I have never even looked at those hoses. I hope that doesn't happen to me.
Glad you found it and got it corrected.
Also, thanks for coming on here and posting your solution as it might help someone else in the future.
Hay 8)
Jimmyt
Mooskee:
Good thing it was #5. You can get to it and #2. The others are between carbs.
dan7uk:
Quote from: Mooskee on August 21, 2025, 12:23:32 PM
Good thing it was #5. You can get to it and #2. The others are between carbs.
Very lucky it was carb #5 easy to get to vac hose and replace, 5 minute job.
Wonder what is involved to get to carb #1, #3, #4, & #6 Air Cut Valve vac hose. I would say the tank would have to come off along with the air box, not sure what else.
Mooskee:
Quote from: dan7uk on August 21, 2025, 07:03:02 PM
Quote from: Mooskee on August 21, 2025, 12:23:32 PM
Good thing it was #5. You can get to it and #2. The others are between carbs.
Very lucky it was carb #5 easy to get to vac hose and replace, 5 minute job.
Wonder what is involved to get to carb #1, #3, #4, & #6 Air Cut Valve vac hose. I would say the tank would have to come off along with the air box, not sure what else.
All that, and remove carb bank. Then split carbs. You would at least have to replace the fuel and air rail O-rings because they will have taken a set from my experience. There is one guy on here that swears O-rings never go bad, but that isnot my experience. Basically if you go that far, you might as well refurbish. Otherwise you might be doing all that work again.
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