¶ 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.
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.
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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