72 LT1 carburetor idle setting

[Fred H.]

Re: 72 LT1 carburetor idel setting

An update on our earlier posted question regarding correct 72 LT 6239 Holley carburetor accellerator pump lid & screws, attached is a photo of the accellerator pump lit & screws from unrestored 72 LT1 (#26496 build dated July 19, 1972). We believe the the 6th edition TIM JD (page 128 fig M 5.3) is incorrect.

#26496 72 LT1 accellerator pump lid.jpg

[Terry M.]

Re: 72 LT1 carburetor idel setting

An update on our earlier posted question regarding correct 72 LT 6239 Holley carburetor accellerator pump lid & screws, attached is a photo of the accellerator pump lit & screws from unrestored 72 LT1 (#26496 build dated July 19, 1972). We believe the the 6th edition TIM JD (page 128 fig M 5.3) is incorrect.

Fred
The worst kind of advice is that which is unsolicited (Not my quote. Someone else's).

I'll break that rule and share what I have learned in over twenty years of writing and revising Judging Manuals.

It is a mistake to assUme that because one finds one example (or even 5 or 6 examples) that all Corvettes of a given year follow that (those) example(s). The more granular the details one is examining the greater the chances that those details changed during the course of production.

[Fred H.]

Re: 72 LT1 carburetor idel setting

Terry, thank you for the advise. I didn't mean to create a controversy. Thanks for your responce.

Fred

[Terry M.]

Re: 72 LT1 carburetor idel setting

QUOTE=Fred Hickey (66564);914804]Terry, thank you for the advise. I didn't mean to create a controversy. Thanks for your responce.

Fred[/QUOTE]

[Fred H.]

Re: 72 LT1 carburetor idel setting

Terry, I'd like to offer that we have enjoyed all the help provided from the NCRS members though out our LT1 renovation.

[Fred H.]

Re: 72 LT1 carburetor idel setting

Hi Gary, I believe I've noticed other posting from you and that your 72 is a survivor? My question is the photo you posted from that 72 survivor? To me the accellerator in the photo looks to be from a much later Corvette?

[Gary B.]

Re: 72 LT1 carburetor idel setting

You are correct. But with all the original paperwork that came with the car, in the first few years of it's life, it was back to the dealer probably 20-times for carburetor problems, poor running problems, Leaking, not running right, excess fuel use, backfires... Letters written to GM complaining. Probably all the typical Holley problems back in the era? Or was it just GM technicians not knowing how to fix them back then?

I bought my 70 LT1 in 1980. Other than gaskets every several years, it's been pretty trouble free. Thanks to Sunoco 110 ?

When I got the 72, I rebuilt the carburetor, did all my tricks of the trade, new gaskets and such. Reused all the same "hard" parts. It ran well and never leaked. And always started good. If it AIN'T broke, don't fix it?

Yes, that was before Revision #6 of the 70-72 TIM&JG. Never went back and looked at the accelerator pump housing.

As time Marches on, people ask questions, we research, we learn more. I rely on known original unrestored cars to look at, to learn from. Sometimes, a restored car will yield some data, sometimes... If you looked at my copy of the 70-72 TIM&JG, and saw all the little sticky notes along the edge, items to update, correct, you would laugh. Not every page, but I'll bet a 100 + pages... The 68-69 TIM&JG probably has more !!

It's not the destination, it's the journey...

Gary B

[Fred H.]

Re: 72 LT1 carburetor idel setting

Hi Gary, I believe I've noticed other posting from you and that your 72 is a survivor? My question is the photo you posted from that 72 survivor? It appears the accellerator in the photo looks to be from a much later Corvette?

[Gary S.]

Re: 72 LT1 carburetor idel setting

which Gary? If me, no, my car is probably not a survivor as I am the fifth owner. It has been converted to side pipes and then back to undercar. However, every thing under the hood was at one time original to include everything but the spark plugs, wires, wiper blades etc. It also probably had a minor accident. Very original but not a survivor

[Fred H.]

Re: 72 LT1 carburetor idel setting

Hi Gary, I might be confused but I thought you posted a photo of the accellerator pump lid from your car. My apologies if I was mistaken

[Fred H.]

Re: 72 LT1 carburetor idel setting

To close out my question regarding the originality & TIM JD correct of 72 LT1 6239 Holley carburetor accelerator lid & screws. Attached are photos of our carburetor with cross head screws and tapered clutch head screws. FYI we will leave the tapered clutch head lid for judging.

Everyone's help was appreciated


FH 2.jpgFH 1.jpg

[James G.]

Re: 72 LT1 carburetor idel setting

Fred,
In case you didn't get it set correctly... I wrote a little easy to follow blurb on setting the fast idle for another person regarding a Holley.

https://www.forums.ncrs.org/showthre...702#post906702

[Fred H.]

Re: 72 LT1 carburetor idel setting

James, I got it thank you

[Fred H.]

Re: 72 LT1 carburetor idel setting

We've been working on idle on our 72 LT1. We started with iginition timing by replacing the vacuum advance canister (B28) & and changing the centrifugal advance springs (to the weaker black springs per Duke William's recommendation), we also disabled the TCS. Set the timing to 32 @ 3000 RPM (with the vacuum advance plugged). We then reconnected the vacuum advance and the results were (26 @ 1150). The car both idles & runs much better.

I understand 1150 RPM is higher than the specifications, but set any lower it dies rough the AC on. When engaging the AC the idle drops to 750 +/ - RPM.

[Duke W.]

Re: 72 LT1 carburetor idle setting

Sounds like you're making progress. Optimum power is achieved with total WOT advance in the range of 36-40. Also you need to set it at a speed above the point of max centrifugal. At what speed do the black springs bring it all in?

The advertised CR is 9:1, but could be up to half a point lower depending on deck height, so you should have no problem obtaining the above range detonation-free on 87 PON fuel, but it might need 89.

Does your car have a curb idle solenoid? I recall my '72 Vega GT had one. If so try configuring it to engage when the compressor is engaged. That way you can maintain idle speed with compressor engaged.

Best idle speed for the LT-1 cam is about 900 with 30-34 degrees of total idle advance with full time vacuum advance, and with the compressor engaged it needs to pull enough vacuum to keep the VAC locked at full advance. Otherwise idle instability and stalling will be a problem.

I believe the service manual speed mixture adjustment procedure specifies the "lean drop" method. This is an emissions setting. You first use the traditional method of max vacuum at your target idle speed, the lean the mixture to drop speed 50 RPM. Since your car doesn't require field emission tire, just use the traditional method and skip the lean drop.

Duke