C3 air condition

[Bob I.]

I'm getting ready to rebuild the air condition system in my car. I can't figure out how the idle speed changes when the air is on. The motor currently won't speed up at an idle. The idle screw on the carb has what appears to be a diaphram on one end, and where it contacts the carb it has a plunger that moves in and out. For all I know it might be missing some thing. The car is a 1970 with manual tranmission.

[Mark #28455]

deceleration valve

That diaphragm thingee is a deceleration valve. When higher than usual manifold vacuum levels are encountered, like downshifting to decelerate, the higher vacuum on the throttle plates pulls against the deceleration valve and allows the throttle plates to close further to decrease engine backfiring. (at least that's what I was told).

They are hard to come by as most were replaced years ago by well intentioned mechanics (AKA: BUBBA), so don't toss it.

Mark

[Chuck S.]

Re: C3 air condition

I don't think there is anything on a 70 that increases the idle speed when the AC compressor is running. It seems like they added another device to the carb to increase idle speed in 71 or 72, but I believe it lasted only a year or two...what after that, I don't know.

The 70 CSM mentions that V8 engines with AC have hot idle compensating carburetors, but that is for mixture rather than speed. Idle speed is adjusted to 700 rpm with the engine at operating temperature, the vacuum advance hose disconnected, and the AC off. In contrast, those cars with automatic transmissions are set at 600 rpm in drive, so 100 rpm drop in idle speed from 700 rpm was not a concern to the engineers.

It wasn't until later that cars were designed to cycle the AC compressor on/off for efficiency, and with smaller engines that required the idle speed be automatically increased when the compressor cut in. I have one that the idle speed increase is a mixed blessing...when waiting at a stoplight with the AC on, the idle speed constantly increases/decreases in a rhymic pattern (it's broke...rather get a root canal than let some maroon get their mitts on it). I keep thinking someone is going to think I want to race (yuk, yuk )

[Jim T.]

Re: C3 air condition

My original owner 70 coupe with 350/300 and factoy four season air conditioning never had anything on the Q-Jet like you have.

[Mark #28455]

390 HP 454 *NM*