1972 idle solenoid

[Michael C.]

Im doing something wrong. engine 350base 200hp 4speed with A/C. Engine idle without solenoid hooked up is 800rpm. Wire to solenoid is hot all the time; is that correct ?? When you hook up the solenoid idle jumps to 1500rpm. You can see the plunger extend. First thought is that I need to adjust the screw on the end of solenoid, but wait not just yet. When I turn the A/C on idle drops to about 1100rpm.The plunger is always extended with current going to it. Is that correct ?? How much adjustment is there to the solenoid screw ?? Im baffled has to how to correct the problem . ie getting rpm to stay at 750-800 with the A/C on or off. Any thoughts ? Thanks Mike Chase

[Chas Henderson #28127]

Re: 1972 idle solenoid

Mike,
If it is anything like my 71 sb with A/C, the plunger should only operate - stick out - for up to 30 seconds. And should operate for about 5 seconds when you turn the engine off to engage the compressor to but drag on the engine to help stop engine run on.

Chasman

[Patrick H.]

Re: 1972 idle solenoid

Chas, it's nothing like a 71. Take it from someone who owns both.

The 72 DOES remain hot all the time. And it's "different" in that you are supposed to use the solenoid to set the idle, NOT the usual idle speed screw on the side of the carb.

What you need to do is this: with the carb solenoid removed (it's the easiest way) screw in the usual idle speed screw on the side of the carb, and bring the idle speed down to 400 or so. Your car might not even run that low; that's OK.

Now look at the carb solenoid. See how it has a nice hex head on it? Reinstall it, and using your open end wrench turn the screw in "clockwise" (the usual "in" way) to set your idle at 800 rpm (or whatever the number is on the sticker that's on your firewall - my car's not here for me to say). You're set! Of course, all this is "assuming" your dwell and timing are correct.

What happens is that when you turn off the car, the solenoid retracts. The carb then drops to 400 rpm (or so) and no longer "diesels" or "runs on." It was the 1972 version of the fix for that.

If 72 is like 71 with AC, your AC clutch will also kick in on shut down to stop your motor, but that I can't verify as my 72 does not have AC.

I hope that helps.

Patrick

[Duke W.]

Re: 1972 idle solenoid

My erstwhile '72 Vega GT was set up the same way. It's an "anti-dieseling" solenoid, and Patrick's recommendation is basically correct. There should be information on setting the "high" and "low" idle speeds in the CSM. The solenoid is continuously energized with the ignition on and de-energized with the ignition off, which basically "starves" the engine of air and prevents run-on when you switch off the key.

The relatively retarded initial timing and lack of vacuum advance at idle in order to achieve emission bogeys causes the boundaries of the combustion chamber to run much hotter than antecedent non-emission controlled engines, which creates hot spots that cause preignition that allows the engine to "diesel" or "run-on" after the ignition was switched off. The greater throttle opening and higher idle speed relative to previous non-emission controlled engines also contributeds to run-on. These ignition map set-ups also increase the tendency for the engine to detonate due to the hot combustion chamber boundaries, reduce torque, and increase fuel consumption.

Another way around this issue is to replumb the engine for full time vacuum advance along with advancing the initial timing to yield about 25 degrees of total idle timing. The OE spec vacuum can that provides 20-24 degrees of advance should be swapped for a can that provides about 16 degrees such as the NAPA/Echlin VC680 (16@16") or VC1765 (16@12"). If your initial is 8-10 the extra 16 from the vacuum can takes it to 24-26, which is about right for a mild engine. SHP engines can use a little more total idle timing - in the range of 26-32.

Duke

[Terry M.]

Re: 1972 idle solenoid

"If 72 is like 71 with AC, your AC clutch will also kick in on shut down to stop your motor, but that I can't verify as my 72 does not have AC."

1972 is NOT like 1971. The "engage the AC clutch to shut off the motor" idea is thankfully a one-year only (1971) feature.

You are right on target with the rest of your suggestions, Patrick. However, using the Chassis Service Manual (as Duke has suggested) tune-up section is the best advice. It gives a step-by-step how-to.

[Patrick H.]

Re: 1972 idle solenoid

Yes, but how many people actually own AND READ a CSM?

Two?

I put the caveat re: the AC system on shutdown because it was outside my "personal experience base." That, and I've read Jack H's several posts on the 71 system. At least with my other suggestions he can get it "close" before going through the tune-up section step by step.

Patrick

[Duke W.]

Re: 1972 idle solenoid

"Yes, but how many people actually own AND READ a CSM?"

As Karl Mauldin used to say on TV: "Don't leave home without it!"

Duke