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Quadrajet Fast Idle Issue
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Supporting Member of 12/20
Picture of Paul And Jaime
Forgive my ignorance here, I will be using descriptions of things when I don't know what there called.

After My 454 warms up and the electric choke fully opens, the engine still idles fast because of a vacuum actuated canister on the opposite side of the choke. There is a small ( Vacuum Relay ?? ) that has wires running into it. The vacuum signal runs into this little valve then out to the canister. The canister pushes the throttle to increase the rpms. When I force the canister arm in and the rpms goes down enough I can here the ( Vacuum Relay??) Click and it shuts off the vacuum to the canister and my engine will idle down to 800 rpms. As soon as I rev the engine the Vacuum Relay will click and push the canister arm out and not let the engine idle back down. I would like to know what those two parts are called and what is wrong.
The fast idle adjustment is correct, the choke works like it should and the shaft on the vacuum canister is not sticky and works smooth. I tried to do a google search but its hard when you have no clue what the parts are called your trying to search for. Thanks all Oh with this unhooked the engine idle is 800 rpms like it should be.
Posts: 140 | Location: Ky | Member Since: 10-20-2020Reply With QuoteReport This Post
Official Barth Junkie
Supporting Member of 1/23
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Carburetors were always a crude attempt at fuel control. Even the more complicated ones like the QJet were not capable of low emissions. By 1980 all the cars in the US had converted to fuel injection to meet emissions standards of the day, but the regulations for trucks did not catch up until about 1990. The auto companies were finally forced into modern designs with electronic ignition sytems and fuel injection that were far more reliable with far lower emissions.

Meanwhile we have the 80's QJets with crude choke systems, seperate systems for idle, intermediate and high power. Accelerator pumps for transition, and power valve for high loads. Even when all are well adjusted, the emissions were still higher than desired. Even though all the cars were fuel injected by 1980, the auto companies stuck with their old junk on trucks through the 80's until they were forced out by regulations.

One of the problems with carbs is when they decelerate with the throttle plates closed, the manifold vacuum was quite high. The engine is still sucking fuel but getting little air. The result was overly rich mixtures. In addition, if the engine had loose valve guides or worn pistons, it would suck in and burn more oil. When the the throttle was reopened you got a puff of nasty smoke. To reduce the effect, many carbs had decel valves that either fed some air while coasting or held the throttle open slightly to feed more air.

The decel valve was supposed to hold the throttle open slightly until the engine slowed and vacuum went down. Some of these decel valves were vacuum driven by a delay valve, some were electrically operated. It appears yours is an electrical version.

This system was also sometimes used to increase the idle speed when the AC was running, to avoid engine stalling from the AC load at idle.

Since yours is apparently not working correctly, I would try disconnecting it. It should then allow the throttle to close properly, albeit a bit sooner. If your AC still works you may need it but otherwise probably not.

Good luck with your dinosaur carb. Mechanic

"98" Monarch 37
Spartan MM, 6 spd Allison
Cummins 8.3 300 hp
Posts: 4992 | Location: Kalkaska, MI | Member Since: 02-04-2011Reply With QuoteReport This Post
Supporting Member of 12/20
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decel valve

Huh, ok so just more emissions stuff. I unhooked it and plugged the line. Thank you
Posts: 140 | Location: Ky | Member Since: 10-20-2020Reply With QuoteReport This Post
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plugged the line

there is value in replacing all the vacuum hoses. it is ten dollars of tubing and an hour of time, and can resolve many predicaments having to do with carburetor control and ignition timing.

Happy Motoring!

1987 Barth 27' P32 Chassis
Former State Police Command Post
Chevrolet 454
Weiand Manifold, Crane Cam, Gibson Exhaust
Posts: 468 | Location: Massachusetts | Member Since: 07-28-2006Reply With QuoteReport This Post
Supporting Member of 12/20
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I agree. One of the first things I done was make a smoke tester and figure out my whole vac system was a sieve.
Posts: 140 | Location: Ky | Member Since: 10-20-2020Reply With QuoteReport This Post
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