72 LT-1 with A/C - rpm drops signifcantly with the a/c compressor on

[Ed H.]

I am a new proud owner of a 72 LT-1 with A/C and 4 speed. When the a/c is turned on the idle RPM drops significantly to the point of the engine barely running without the automatic kick-up in RPM. The spark control solenoid is engaged when the a/c is off and disengages when the a/c is on. This seems opposite to my thinking however it is called a spark control solenoid which may serve to control dieseling. I am new to the Holley 750 operation. Is there another solenoid or RPM kick-up device that should advance the idle with the a/c compressor on?

[Bob H.]

Re: 72 LT-1 with A/C - rpm drops signifcantly with the a/c compressor on

Hi Ed

Congrats on the purchase of such a rare car.

[Gary H.]

Re: 72 LT-1 with A/C - rpm drops signifcantly with the a/c compressor on

Not familiar with holley or tcs.

Rochester on my 69 427 has an idle solenoid for the ac. The plunger does not really have the power to raise the rpm from idle. It does however have the ability to maintain a higher rpm once engaged. Try to kick on the ac while racing the engine to 1700 or 1800 rpms and then let off the gas after a second or two and see if the solenoid is maintaining the higher rpm level.

[Bob H.]

Re: 72 LT-1 with A/C - rpm drops signifcantly with the a/c compressor on

Ed

I believe the air cars had an idle control solenoid on the front left side of the carb? Do you have any pics?

Bob

[Duke W.]

Re: 72 LT-1 with A/C - rpm drops signifcantly with the a/c compressor on

You need to audit your configuration relative to the AIM, and there may also be information in the CSM.

Anti run-on solenoids were common in that era along with relatively high idle speeds. If there is no specific "idle-Up" solenoid when the compressor is engaged, set the idle speed/mixture at the lowest RPM that will produce an acceptable idle with the compressor engaged, and you'll just have to accept a higher than necessary idle speed when the compressor is not engaged.

I recall my '72 Vega GT four-speed had an idle specification of 1200 that you set by turning the solenoid plunger, and a low idle speed of 700 with a convential idle adjustment screw on the throttle lever, so when you turned-off the ignition the solenoid de-energized, which closed down the throttle plate to prevent run-on. Run-on was a problem because the ported vacuum advance increased EGT and cylinder boundary temperatures, which could cause preigntion run-on with the high idle speed, but the higher EGT at idle was necessary to encourage oxidation reaction in the exhaust with or without AIR. Higher idle speeds also reduce emissions.

It was a kluge, so not long after owning it I converted the ported vacuum advance to full time and redid the spark advance map with a different VAC and centrifugal curve and placed a piece of electical tap over the idle solenoid spade so it couldn't energize. It was noticeably quicker, ran cooler, got better fuel economy, idled nicely at 700 and never ran-on. It didn't have A/C, so that was not an issue, but if it did I would have just wired that solenoid into the compressor clutch circuit and adjusted it to maintain 700 or a little higher with the compressor engaged.

That's just the way is was back in the day of crude emission controls. A lot of engine configurations were kluges. We're spoiled by modern digital engine management systems that make the engine run perfect in all conditions, and you virtually never have to lift the hood.

Duke

[Ed H.]

Re: 72 LT-1 with A/C - rpm drops signifcantly with the a/c compressor on

Thank you Bob, Gary and Duke. After much reading and double checking the carburetor on the 72 LT-1 it appears to not have an a/c idle kick-up solenoid. The solution is as you suggest Duke and if I want to continue with stock from the day it was delivered I will have to accept the "kluge" of crude emission control and higher normal idle. Here are of the couple of pictures of the LT-1 carburetor and engine.

photo 1.jpg

photo 2.jpg

[Roy V.]

Re: 72 LT-1 with A/C - rpm drops signifcantly with the a/c compressor on

Welcome to the NCRS, nice rare LT-1 you have, approx 240 made, I have a 72 as well, sadly no A/C.
a picture or two of your car would be nice

Regards
Roy

[Gene M.]

Re: 72 LT-1 with A/C - rpm drops signifcantly with the a/c compressor on

Ed,
Why does the inside of your Holley look so black carbon?

[Edward M.]

Re: 72 LT-1 with A/C - rpm drops signifcantly with the a/c compressor on

I have been down this road with a number of C3s of the years. I had a 71 coupe, auto, A/C that had about a 100 rpm drop when the A/C was turned on.I had a 73 convertible, auto, A/C that had about a 400 rpm drop with the A/C on. The 73 would not keep running when set at the factory idle setting.I had to bump the idle on the 73 a bit. That lasted until the compressor bearing seized on day. I replaced the entire compressor and hose assembly (the hose was looking kind of ratty), and the problem disappeared. The rpm drop with the rebuilt compressor was between 150 and 200 rpms. I suspect that there is a specification for how much the rpm should drop when the A/C is turned on.

[William F.]

Re: 72 LT-1 with A/C - rpm drops signifcantly with the a/c compressor on

What's that in front of where the throttle cable attaches to carb? Not an idle solenoid? As someone said, if it doe have one, you have to stab the accelerator to get it engaged where it will hold the idle up..

[Michael W.]

Re: 72 LT-1 with A/C - rpm drops signifcantly with the a/c compressor on

What's that in front of where the throttle cable attaches to carb? Not an idle solenoid? As someone said, if it doe have one, you have to stab the accelerator to get it engaged where it will hold the idle up..

That's the anti-dieseling solenoid activated as soon as the ignition is turned to the run position.

[Patrick H.]

Re: 72 LT-1 with A/C - rpm drops signifcantly with the a/c compressor on

That's the anti-dieseling solenoid activated as soon as the ignition is turned to the run position.

No, it's not. On a 1972 it's the idle solenoid, and it's used to set the idle. It also acts as an anti-dieseling solenoid by cutting power and dropping the idle to 400rpm when you turn off the car; however, it must be used to set the idle speed of the car. This is different from other years. I do wonder if this particular car is not correctly set, and that is why he has some problems with idle speed when the AC is turned on.

[Michael W.]

Re: 72 LT-1 with A/C - rpm drops signifcantly with the a/c compressor on

This is different from other years.

'No, it's not'. Jeeze..........

Same as '73, '74 etc.

[Patrick H.]

Re: 72 LT-1 with A/C - rpm drops signifcantly with the a/c compressor on

'No, it's not'. Jeeze..........

I'll be sure to swing the 2x4 a little harder next time so I get your attention.

Same as '73, '74 etc.

That would require me paying specific attention to the configuration of a post-72 Corvette, at least with regard to the carb setup and solenoids. I don't see that happening anytime soon.

[Wayne P.]

Re: 72 LT-1 with A/C - rpm drops signifcantly with the a/c compressor on

Here are a few pictures I took at KC to help out a friend who bought my 72 LT1 w a/c in 1985. It looks like your are missing the same solenoid as my friend. I have more pictures on another camera that I have not looked at yet. If you can contact me, I can put you in touch with the two owners. I can post more pictures or send you what I have. Also inside of my friends Holley is black like yours.
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