C-3 1970 coupe

[Jack bogosian]

When I put the a/c on the idile drops to around 600 and the car run ruff. Is the rochester 4 barrel carb have a AC step up solinoid wire that may have fallen off Thanks in advance JAck

[Joe L.]

Re: C-3 1970 coupe

Jack-----

Yes, your car should have an idle control solenoid. It will be mounted on the drivers side of the carburetor. The wire may have come off or the solenoid may be inoperable. There could also be an adjustment problem.

[Tom R.]

Re: 1970 side louvers

Jack, to the best of my knowledge the carb solenoid set up was not used on the 70 model. I have a BB with a/c & auto, the best I have done is make sure timing is correct & idle mixture is set correct, then adjust the carb idle speed with trans in drive. LOL

[Jim T.]

Re: C-3 1970 coupe

Jack and Joe, I have a original owner 70 coupe with factory air and it did not come from the factory with a idle control solenoid. Does have the TCS solenoid that controls idle speed with its own temperature source for input. Does this by allowing vacuum to the distributor vacuum advance unit full time to help bring down engine temperature. Discovered this feature back in Sep 1970 when the car was parked with the air on one day. Few days later I converted my new car to full time vacuum advance with a new piece of vacuum hose and that is the way it is today.

[Jack H.]

Guess, I missed it...

My recollection of '70 cars, is that there's no carb idle solenoid (that started in '71 with the 1-year only CEC system and continued into '72 and beyond) and the only 'solenoid' used was the TCS solenoid that controled vac advance to the distributor. Further, I recollect no difference between A/C and non-A/C engines in terms of the triggering of the TCS solenoid in '70.

In '71, the CEC solenoid DID 'step' the carb's curb idle as well as 'toggle' the vac advance to the distributor, but there was NO DISTINCTION in the system's control algorithms to discriminate between A/C and non-A/C equipped engines....

The '71 cars equipped with A/C DID have a set of compensating control relays (mounted in the cabin on the Harrison air box), but their only purpose in life was to over-ride the center console A/C controls and FORCE the clutch on the A6 compressor to fire on engine shut-down to add 'drag' and thwart diesel run-on. The Chassis Service Manual gives a theory of operation 'snapshot' here, decribing the relay system used as a compensation for the "wider throttle angle required by A/C equipped engine".

The Catch-22, is the tune-up decals and instructions do NOT distinguish different idle RPM settings required for A/C vs. non-A/C equipped cars! I can guarantee you, if you set up a '71 A/C equipped car following the 'book' in terms of curb idle, it'll have a propensity to stumble and stall when the A/C in engaged UNLESS you tweak the carb's curb idle UP about 100-200 RPM over what works for a non-A/C equipped car...

Two bottom lines here: (1) there was NOTHING special done to 'toggle' the curb idle RPM for A/C vs. non-A/C equipped engines in terms of 'tricks' regarding the vac advance to the distributor ('70) or the state of the carb idle solenoid ('71) based on whether or not an A/C car was running the A/C with the A6 compressor engaged...

Apparently, this WAS seen as a problem on '70 cars, since the system was changed in '71 to add 'smart relays' on the A/C system that over-rode the A/C controls in the cockpit to FORCE the compressor to engage during engine shut-down and add drag to compensate for an "increased carb throttle angle" to thwart engine run-on.

My hunch is, it's a situation of getting the curb idle set 'just right' in '70 for the engine not to 'race' with the A/C OFF and not to 'stumble' with the A/C ON during warm engine idle. Further, I'd venture some of this depends on whether the A6 compressor is, indeed, the real McCoy Corvette model designers intended with it's smaller 10.8 cu-in displacement, vs. the Chevy passenger car model NAPA and the rest of the aftermarket world sell for the application (one fits all) with a 12.6 cu-in displacement. The larger displacement A6 compressor would seem to me to HAVE to have a higher engine drag profile...