1972 Idle with A/C on

[Jerome P.]

The engine on my '72 LT1, after warm up, idles at a steady 850 RPM's. However, as soon as I turn the a/c on the idle RPM's drops off enough that the engine wants to kill. To keep the engine running I either have to turn the a/c off or manually increase the RPM's via the accelerator. I know the a/c will drawdown the RPM's some, but how much I do not know. Any ideas as to what is going on and how much of a drop in RPM's is typical?

[Duke W.]

Re: 1972 Idle with A/C on

850 is a low speed to achieve a stable idle on the LT-1 even without C60. I recommend 900.

Unless the engine has an "idle up" solenoid that engages with the compressor to maintain acceptable idle speed, your only choice is to increase idle speed.

You may find some guidance in the service manual or AMA specs. Otherwise with the compressor engaged set an idle speed you find acceptable, which I'll estimate will be no less than 800. It's a subjective criterion, so you have to make the call. Then go through the idle speed/mixture adjustment procedure using the pre-emission technique without going into the "lean idle drop" procedure that was often used with early emission controlled engines.

Then with the compressor off you have to just accept whatever the idle speed will be - probably 1000-1200.

You might also find that that you can achieve better idle quality by converting the ported vacuum advance to full time, but you will need to replace the OE VAC with a B28 or don't even try.

Duke

[Jim T.]

Re: 1972 Idle with A/C on

Jerome I don't have a LT-1, but I have had my 1970 C60 since new and vacuum advance was changed to 100% full time in less than the first month of ownership. Improved gas mileage in city driving in Dallas.