¶ 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.
 Deceleration Diaphragm |
 Deceleration Diaphragm Plunger |
 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 |
 Decelerator Diaphragm Vacuum Port |
 Decelerator Air Jet # 1 |
 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.
 Idle Jet Passages |
 To Idle Jet Ports and Passages |
 # 1 Deceleration Diaphragm Plug |
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