I have been working on the carb on my sons car(83 camaro had LG4) for about 2 weeks now on and off and I just can't get the thing to Idle

350 engine recently rebuilt with flat top pistons, World Product SR heads ported from bowl to gasket. Mild cam .450 lift Quadrajet (1983 off original 305)carburator Recently rebuilt

The car runs great above idle with good throtle response. But when I try to set the idle the tach bounces between 750 and 850 rpm. Adjusting the idle mixture screws does not seem to have any affect.

With the idle set at 850 to 950 when I put it in drive it just stalls.

Also at low rpm you can hear a backfire in the catylitic converter or the exhaust system someplace.

What is causing this I need help it is driving me nuts.

 

Check secondaries to make sure they are fully closed. Look for anything that would cause low vacuum (leak). Idle is probably too high for mixture scew adjustment to be effective. Push on TPS pushrod
with a feeler gauge while at idle. Does engine stumble when
you do this? Is mixture solenoid operating (clicking when ignition
switch is turned to "on"? Does fuel drip from primary nozzles
at "idle"?

 

This carb is not computer controled. The only thing electrical on it is the choke and the anti dieseling switch.

I have looked everywhere I can for a vacuum leak and unless I have missed it there are no leaks.

could this be caused by bad plugs or plug wires?

 

Ratchet,

You may have a vaccum leak or not. Just for grins, disconnect the power brake booster vacuum line and cap it while you diagnose the problem. Remember to reconnect it before driving.

You may have a gasket leaking between the air horn/upper carb body and the bowl, allowing some unintended air bleed in the idle circuits. You may also have the notorious leaking well plugs at the base of the carburetor. Either of these will render the idle mixture adjustments useless. Another possibility is a varying float level.

I would tend to believe you don't have a problem with weak ignition or plug wires, since that would normally manifest itself at higher RPMs as well. However, if the plugs and wires aren't in a known-good condition, you might want to at least test the wires and inspect the plugs. The plug condition may give you some hints about the mixture as well.

 

Thanks for the help Vader!

I had to go to Italian class tonight so I did not have time to work on the car. After work tomorrow(Tuesday I will try plugging the Brake Booster line)

I also purchased a new set of wires today because I remebered that My son had recently changed the plugs. ( you don't think he pulled one of the wires apart when he was taking the plugs out do yah.) At any rate I will do each test seperately and let you know what happens.

Thanks again
Ric

 

Oh yeah one of the other symptoms of this problem is that there is a strong vacuum signal going to the distributor at idle. This also makes it difficult to get the darn thing to idle.