72 LT1 carburetor idle setting

[Fred H.]

In adjusting our 72 LT1 AC 4 speed what are the correct idle speed rpm (after warm up & choke is open) and the cold start (choke closed) fast idle rpm. Presently with the air/fuel mixture set, our warm idle is 900 rpm & the cold start idle is 1700 rpm

[Owen L.]

Re: 72 LT1 carburetor idel setting

In the '72 GM specs, hot idle is 900; I can't find a cold start spec. 1700 seems a bit high as I'd expect more along the lines of 1300-1500.

[Ed H.]

Re: 72 LT1 carburetor idel setting

Owen and Fred, actually 1700 RPMs is not high enough for cold start fast idle. For a 72 LT-1, with TCS operating and ambient engine temp below 80 degrees and the choke should be set at the high step of the fast idle cam, the fast idle should be at 2350 RPMs. I know it sounds crazy but that is the correct factory spec.

Fred the curb idle should set to 900 RPM after the car warms up. That said the C-60 option GM A-6 compressor will pull down your RPMs by 100 to 150 RPMS and for summer driving I raise my curb idle to 1000 to 1050 RPMs as my 72 LT-1 does not like it much with the AC on and set at 900 RPM curb idle.

I hope this helps and happy to answer any further questions that you may have.

Ed

[Duke W.]

Re: 72 LT1 carburetor idel setting

Most fast idle specs are for a fully warmed up engine, and then upon a cold start it will be 200-300 revs less. For example my SWC's 340 HP engine's hot fast idle spec is 1750, and on a cold start it usually stabilizes at 1500 within a few seconds of firing.

Emission controlled engines generally have higher recommend hot curb and fast idle speeds.

Duke

[Fred H.]

Re: 72 LT1 carburetor idel setting

Hi Ed, thanks for your reply. Are the cold start fast idle & curb idle rpm #'s you referenced from the 1972 Corvette (Chevrolet) Service Manual?

[Ed H.]

Re: 72 LT1 carburetor idel setting

Fred, yes they can be found on pages 14 and 15 of the specification section for Holley carburetors in the back of the 1972 Chevrolet Service Manual. NCRS also lists this information in the Corvette Operations and Performance Verification test Guide for 1968 to 1972.

[Fred H.]

Re: 72 LT1 carburetor idel setting

Thanks. Question the (4th gear timing advance) TCS should not affect these caburetor setting (when the car is in neutral), correct?

[Ed H.]

Re: 72 LT1 carburetor idel setting

Fred, on a 72 LT-1 the TCS controlled spark advance allows vacuum advance when the car is 4th gear as you stated and also when the engine head temp is below 80 degrees. The 2350 fast idle spec assumes the vacuum advance is engaged (engine head temp below 80 degrees at start up). When the weather is warm but the below 80 degrees the 2350 fast idle will drop somewhat quickly (about 100/150 RPM) as the head warms up and once gain TCS blocks the vacuum advance. The good news is the 72 LT-1 uses direct manifold vacuum rather than ported vacuum for the vacuum advance but the bad news is the TCS controls it either by the transmission being in 4th gear or the engine head temp being below 80 degrees.

The fast idle spec for the 72 LT-1 is with the head temperature below 80 degrees and vacuum advance engaged.

Ed

[Patrick H.]

Re: 72 LT1 carburetor idel setting

Fred,

The specs should also be on the white sticker above the driver's side hood latch on the firewall.

Click here to see:

https://flic.kr/p/rwuWA7

[Daniel T.]

Re: 72 LT1 carburetor idel setting

How do you adjust how quickly it comes off cold idle? My 72 LT1 takes forever to come down to warm idle, occasionally jumping back up to 1500rpm idle on a cool day (50-60 degrees) even with the water temps at operating temp? Do you bend the kink on the lever coming off the divorced choke coiled flat spring/thermostat? thanks!

[Fred H.]

Re: 72 LT1 carburetor idel setting

Hi Patrick, the LP emission label on our 72 LT1 doesnt address the fast idle rpm

LT1 emission lable.jpg

[Fred H.]

Re: 72 LT1 carburetor idel setting

Got it, thanks for the clarification. On a different 72 LT1 (6239) Holley carburetor question, we're researching the correct accelerator pump cover & screws. We received a minor deduction for having incorrect screws (annotated should be clutch head), the 6th edition TIM JG (page 128 fig M5.13) shows an example of the clutch head screws). While in the process of procuring a correct accelerator pump lid for tapered clutch head screws, we became aware that on some 72 LT1 (6239) Holley carburetors they used a cover with cross head machine screws. By chance would you know what type of screws your 72 LT1 carburetors accelerator pump cover had? see photo of our cross head machine screws


72 LT-1 Holley Accelerator Pump Screws 101.jpg

[Gary S.]

Re: 72 LT1 carburetor idel setting

My early LT-1, #6421, has the same Phillips or cross head screws. It has been that way for all of my 22 years of ownership. 30300 miles when I bought the car.

Gary

[Fred H.]

Re: 72 LT1 carburetor idel setting

Thanks for your reply. Our late 72 LT1 #6756 also had cross head screws. However the 6th Edition TIM JG is showing clutch head screws??????

[Ed H.]

Re: 72 LT1 carburetor idel setting

Daniel, there are two things that control the time it takes to come off fast idle after the choke vacuum pull off. The temperature sensitivity of the divorced choke bi-metal spring and the length of the rod from the coil spring to the the choke operating lever (choke rod qualifying adjustment). The butterfly valve once the choke is set should just be closed at room temp (72 degrees).

Ed