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Old 01-09-2010, 02:08 AM   #1 (permalink)
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can anyone tell me what the air cutoof valve does and why?
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Old 01-09-2010, 02:27 AM   #2 (permalink)
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this is from another post same question
twisty wrote
Stevo, The air cut-off valve cuts the (vent) air flow off to the pilot jet circuit on decel or a high vacuum condition...

Basically when you are riding along at part throttle, then drop the throttle (close the throttle suddenly) the engine manifold vacuum goes high, at the same time the carb throttle valve closes so that means the air flow in the carb/intake manifold drops to almost nothing. What happens then is the remaining mixture in the manifold goes very LEAN... What the air-cut-off valve (or valves on early Wings) does is momentarily shuts the air supply off to the pilot jet's air side (blocks off the pilot jet air jet) so the decel manifold mixture goes somewhat rich, that rich mixture is there to reduce the popping out of the exhaust system on decel or high vacuum over-run conditions. There are about 3 ways the air cut-off system can fail.

(1)- the cut-off valve diaphragm starts leaking.. That causes a lean condition on decel or erratic operation of the cut-off valve..

(2)-the air cut-off valve diaphragm gets old & hardens up. That can cause all kinds of problems as it now won't react to the vacuum signal correctly & can cause either low air flow to the pilot jet circuit & therefore a rich carb mixture (poor mileage, fouled spark plugs, probably won't need the enrichener valve for more than a few seconds at start up),, or a lazy valve so you don't get proper decal air cut-off.

(3) a plugged air inlet to the idle air cut-off system. That will give a rich carb mixture (poor mileage, fouled spark plugs, probably won't need the enrichener valve for more than a few seconds at start up).. No air to the pilot jets really richens the low speed mixture & gives terrible fuel economy, rich blubbering at low throttle settings, bad exhaust smell at idle, etc.

That valve can be checked with a vacuum gauge on the signal vacuum line. I don't have the vacuum numbers handy for valve operation parameters but believe it's somewhere in the 16"hg range. (the numbers are in the manual). The problem is; those cut-off valves can hold vacuum OK & even seem to work but if the diaphragm is old & hard it won't operate at the correct vacuum line signal. On an old Wing it's best to just replace the cut-off valve's diaphragm & spring (not much money from someplace like Bike Bandit).

If your engine idles OK & doesn't pop in the exhaust when decelling your cut-off valve is probably at least working. You just don't know if at the corrrect vacuum signal.

Your mileage drop could be to many things from a plugged air filter, to carb problems (those carb floats are real critical to height for economy), to a plugged idle air cut-off air supply, to misadjusted pilot jet needles, to someone before you re-working the carbs for smoother performance, to a weak ignition, to many other possible causes. About all you can do is go through everything one at a time & verify all is OK..

Twisty
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Old 01-09-2010, 03:18 AM   #3 (permalink)
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Thanks my bike idles fine but have a super lean condition on acceleration have had the carbs off twice not sure where to go from here.
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Old 01-09-2010, 03:54 AM   #4 (permalink)
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not much on carbs. but if it's lean you sure you got everything clean? vaccume leaks? try running some seafoam in it a whole can.



someone will be along maybe in the morning now. oh it is morning.
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PGR # 190570
cc permit
1988 1500 trike to be
1980 cb900 custom
gl 1988 1500 trike sold
2003 flhtuci sold
gl 1986 1200 trike sold
gl 1984 1200 trike sold
first bike cb 160 honda
to many to list in between
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Old 01-09-2010, 08:51 AM   #5 (permalink)
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Try squirt some starter fluid at possible air leak places if the engine jumps you've found your problem..intake a likely place to start.
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Old 06-09-2011, 11:03 AM   #6 (permalink)
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How did you access the valve from the front behind the radiator? The air cut-off valve is on the underside of the carb rack at the rear. A similar looking valve is at the front.
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