Deceleration Air Valve

Deceleration Air Valve


Your running along, doing OK and you shut off the throttle for a Stop sign and the exhaust goes pop, pop, and bang. Noisy and irritating. With the EPA pushing leaner and leaner jetting this popping was getting to be a real problem. The popping is caused by a lean condition in the exhaust. The cure is to add just a bit more fuel while the throttle is closed. We do this with an Air-Cut Out Valve. I like to call them a Deceleration Air Valve.

Under normal running conditions the two Deceleration Air Jets are both allowing air to go to the idle jet ports(s). When you close the throttle a high vacuum is produced in front of the throttle slide. This vacuum sucks the Deceleration Valve diaphragm open causing it to block air from the top (# 1) Deceleration Air Jet. Air is still drawn from the lower (# 2) Deceleration Air Jet. This makes the mixture coming from the idle ports richer and that stops the back fire in the exhaust.

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Deceleration Diaphragm
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Deceleration Diaphragm Plunger
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Deceleration Diaphragm,
Spring and Cover


In normal running there are two Deceleration Air passages. The top, # 1, Deceleration Air Jet is bigger then the lower, # 2, Deceleration Air Jet. In this example, the biggest Jet Drill I was able to put in the # 1 Air Jet was a # 64 drill (.036"). The biggest Jet Drill I could get into the # 2 Air Jet was a # 77 (.018"). The # 1 Air Jet is about 75 % bigger, in area, then the # 2 Air Jet. That means when you cut out the # 1 Air jet, you have cut out a lot of air.

Below I put one tiny copper wire in the # 1 and # 2 Decelerator Air Jet ports and the Diaphragm Vacuum port to show you where they are.


Decelerator Air Jets
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Decelerator Diaphragm Vacuum Port


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Decelerator Air Jet # 1
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Decelerator Air Jet # 2


These Air Jets and passages usually stay clear and trouble free. However, the Diaphragm plug can corrode to the Carburetor body or the Diaphragm its self can develop a hole. Some people say to plug the Deceleration Air Valve with Silicon Seal but I really don't see a need. If it is working just leave it alone.

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Idle Jet Passages
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To Idle Jet Ports
and Passages
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# 1 Deceleration
Diaphragm Plug






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