1972 - LT1 with Air Conditioning - Curb idle

[Jerome P.]

I have the curb idle set at 1000 RPM's with the A/C off (recommended is 900 RPM's with the A/C off). The cold idle after starting is initially at 1700 RPM and as the engine warms to 180/190 degrees the idle drops to 1000 RPM's. However, as soon as the A/C is engaged the RPM's drop to 500 and the engine struggles to run and would kill if not for working the throttle and increasing the engine's RPM's.

What am I missing?

Any help will be appreciated.

Jerry

[Jerome P.]

Re: 1972 - LT1 with Air Conditioning - Curb idle

Just renewing my post. Again any thoughts regarding my previous post would be appreciated.

Jerry

[Matthew S.]

Re: 1972 - LT1 with Air Conditioning - Curb idle

Jerry,

I also have a 72 LT-1 with AC and I had a similar problem. My car would idle OK but when I turned on the AC it would stall out. What I did was readjust the points dwell (it was off about 6 degrees) and then I readjusted the timing (that was off a few degrees as well). Once I did that I reset the idle when warm to 900 rpm and when the AC is turned on the car idles at about 650 - 700 rpms and runs fine.

Good luck - Matt

[Duke W.]

Re: 1972 - LT1 with Air Conditioning - Curb idle

I don't know the details of your configuration, but I would think that GM would have had to install an "idle-up" solenoid that cracks the throttle open slightly when the AC compressor is engaged. High overlap cams can't handle much additional load at idle and maintain idle stability.

Prior to reindexing the cams on my Cosworth Vega to significantly reduce overlap the idle could destabilize merely by turning on the headlights.

Make sure everything is there, functional, and properly adjusted according to the CSM and AIM.

Duke

[Jerome P.]

Re: 1972 - LT1 with Air Conditioning - Curb idle

Matt, After readjusting the dwell and timing you said you reset the idle. Did you set the idle at the "idle stop solenoid" or on the carb?

Jerry

[Matthew S.]

Re: 1972 - LT1 with Air Conditioning - Curb idle

Jerry,

You set the idle with the idle stop solenoid, not the idle screw on the carb. The idle screw on the carb should not be used. When you turn your car off the idle stop solenoid plunger will release, the idle will drop way down and the engine will shut down. If you adjust the idle to 900 rpm (which is the recommended setting) using the idle screw on the carb the engine will diesel when you turn off the car because the idle is too high. The idle stop solenoid allows the engine to run at a higher rpm (so you can engage the AC) but then turn off without dieseling.

Hope that helps.

Matt

[Jerome P.]

Re: 1972 - LT1 with Air Conditioning - Curb idle

Matt,

I am at my Witt's end. I have done everything you suggested and the service manual says. The engine still struggles to run when the A/C is engaged.

As I mentioned before the carb was rebuilt by Holley.

Do you thing if I increased the jet size; it would help?

Also, are you aware if the 1972 LT1 engine configuration with A/C had a "kick-up solenoid? I have not found that it did, but thought I ask.

Again your thoughts would be appreciated.

Jerry

[William C.]

Re: 1972 - LT1 with Air Conditioning - Curb idle

Have you tried dropping the idle to 900? That may sound counterintuitive, but the higher the idle, the more you move away from the idle circuits in the carb and get into the main circuit. The main circuit will not get enough signal at the lower rpm caused by the load of the compressor to feed properly. This assumes all other aspects of the tuneup are 100%, incliding the distributor and especially the vacuum advance system. Are the valves set properly or a tad on the loose side? that is also critical in maintaining the low speed torque of the engine.

[Dave S.]

Re: 1972 - LT1 with Air Conditioning - Curb idle

My 71 base motor TH400 car with A/C idles at 550 RPM and drops to 475-500 RPM when the A/C is turned on. I know it is a Q-Jet car but does that make much difference. ??? I have the TCS/CEC set up so there is no solenoid control of the idle. What happens to this car follows the thought that Bill made.

[Dick W.]

Re: 1972 - LT1 with Air Conditioning - Curb idle

Clup, what do you think about opening the secondaries slightly, there by allowing the primary throttle plates to be closed slightly more. That would assure that they are not opening past the idle transfer slots.

[William C.]

Re: 1972 - LT1 with Air Conditioning - Curb idle

I'm not sure I'd try anything that permanent until all the straight foreword approaches to making it absolutely "Stock" have been addressed, including timing map. vacuum advance and the cam timing/lash. Everything is inter-related in the engine at low rpm, and the factory versions when new did not exhibit this problem, so it should be managable without moving outside the OEM framework. even slight loosening of the lash and dropping the idle to stock spec would be major from the standpoint of the carburetor system. If not, then more drastic action but I remember one car like this that primarily had an issue with the distributor curve, I fixed the curve and the problem went away.

[William C.]

Re: 1972 - LT1 with Air Conditioning - Curb idle

another thing to watch in looking at the distributor specs is that the 1000 rpm idle is about where the distributor mechanical advance starts to kick in. if this is the case on yours (the factory spec gives some latitude as to the actual start), and the car is tuned at that idle, the rpm loss when the A/C pulls down the idle will also pull timing out at just the point you do not want it to. The factory spec is 0-4 degrees (crank degrees) at 1350, and for use with A/C, I'd shoot for the low end of that range at 1350 just to stabilize the idle when A/C load shifts on and off.

[Duke W.]

Re: 1972 - LT1 with Air Conditioning - Curb idle

Here's another strategy. Convert the vacuum advance to full time and if installed bypass TCS and run initial timing such that the total initial plus centrifugal is 36-38 degrees. You will also need to install a NAPA VC1810 VAC or equivalent in another brand (stamped B28).

Set the idle with the carb idle adjusting screw at about 900. Wire the solenoid to the AC compressor clutch so it only deploys when the compressor is engaged and set the idle speed at 900 with the compressor engaged and the solenoid deployed.

With full time vacuum advance and about 14 degrees initial timing, total idle timing will be about 30 degrees and the engine will not run on when shut off. The reason it does with the OE setup is the hot combustion chamber surfaces that develop due to the lack to total idle timing that the engine wants for minimum EGT.

Duke

[Joe R.]

Re: 1972 - LT1 with Air Conditioning - Curb idle

Double check to make sure that your vacuum advance is not dropping out when the engine load increases from the A/C. If the engine vacuum under the A/C load drops below the point necessary to keep the vacuum advance fully cranked in, the drop in the advance will cause a drop in RPM and a drop in idle smoothness. If you find that your vacuum advance is dropping under this condition, you can install a vacuum can that has a lower threshold. There is much discussion about this topic in the archives.

I should note that the above comments assume your engine has full manifold vacuum applied to the distributor at idle. If you have the emissions-related "ported vacuum" that reduces the vacuum at idle, it might be a good idea to switch to a source of full manifold vacuum.

[Bill C.]

Re: 1972 - LT1 with Air Conditioning - Curb idle

Hey --

Just a thought ... With the CEC system functioning, there is not any vaccum available for advance until you get into 4th gear.
You should be operating on mechanical advance only at idle and until you are in 4th gear - OR - the car is operting at a temperature under 72 degrees F.

Maybe you need to chaneg the advance weight springs or check to see if the advance plate is turning freely.

Not an expert on this subject but remember these items from my car.