72 LT1 carburetor issues

[Fred H.]

We're experiencing issues with our 72 LT1 6239 Holley carburetor. Specifically, the problem is maintaining consistent both fast idle and curb idle (and an occasional backfire)

[Dan A.]

Re: 72 LT1 carburetor issues

It took years to get the carb on my 72 LT-1 right. Two different “greatest Holley guys in the world” (as recommended on another forum) did nothing but charge me a lot of money and treat me like dirt. When I finally retired I had more time, so I bought a Holley trick rebuild kit and spent a lot of time trying different jets, squirters, secondary springs, power valves and accelerator pump cams and now it runs great. None of that answers your question, but usually people chime in with some Holley rebuilders that they have good experiences with. The correct vacuum advance canister also made a lot of difference. In all fairness, I have found that there are a lot of variables like elevation, climate and available fuel that can’t really be dialed in on a test engine.

[Fred H.]

Re: 72 LT1 carburetor issues

Your candid response is greatly appreciated. We're not neophytes regarding rebuilding carburetors, but we believe that just taking the carburetor apart, cleaning it and reassembling it with new gaskets isn’t the answer. The previous owner had replaced the original 6239 Holley with a Demon double pumper (original 6239 Holley saved in a box) for an apparent issue.

If you don’t mind me asking Is this the Holly trick rebuild kit you referred to?
https://www.holley.com/products/fuel...t/parts/37-933

We see this kit has various gasket, jets / metering components, accelerator pumps & springs. What was the process you went through in finally getting your LT1 performing correctly? We know there is Holley Carburetor Handbook for 4150’s.

[Dan A.]

Re: 72 LT1 carburetor issues

Fred,

That is the kit I have. I understand your concerns that it may need more than just some parts renewal/swapping and you might very well be right. In my case I had two major issues. One was a flat spot at throttle tip in and the other concerned the secondaries opening too quickly. It was a matter of trial and error to determine which combination ran best. I also changed the VAC to a B28 per Mr. Williams suggestion and that by itself helped the driveability. Again, there are a lot of variables and I am far from an expert in this area. Maybe I just got lucky. I also sent you a PM with the two people that I had no luck with - didn’t think it was proper to disclose that on a public forum.

[Timothy B.]

Re: 72 LT1 carburetor issues

Fred,

After reading your post my best guess is that the carburetor surfaces are not flat. These Holley's are great carburetors if the surfaces are sealed tight but in time with repeated heat/cool cycles they can leak internally both air and fuel.

Tear it down and have a machine shop check and mill the main body front, back and bottom along with the throttle plate top and bottom. You may even need to replace the metering block because it can also be warped. When I overhaul one I check it with a miti-vac through the power valve feed hole and if there are leaks the vacuum gauge will fall to zero very fast. No leaks and the gauge will still fall but slow because air will get sucked through the gaskets or screw threads etc but that's pretty much normal.

Only mill enough to flaten the surface or you will have other issues.

[Peter G.]

Re: 72 LT1 carburetor issues

Does Holley have a tech help line? It's my understanding that most carbs need to be re-jetted to be more compatible with today's gasoline blends. Gas in 1972 was very different from today's gasoline, especially with its molecular weight. Today's gasoline atomizes differently than 50 years ago. Perhaps different number size jets would work best for the fuels that are currently available. Holley will know.

[James G.]

Re: 72 LT1 carburetor issues

1972's are leaner than 1970 and 1971 using 68 jets in the primaries.
IF you use a gasoline with ANY ethanol what so ever it will be a lean(BTU) mixture due to any ethanol blend being lower in overall BTU's per gallon than a straight gasoline, elec fuel injection and knock sensors can compensate for this, carbs cannot.

I agree with everything above regarding flatness of mating surfaces etc, I have had badly warped fuel bowl rails, warped metering/jet plates, and warped mainbodies on Holleys. IF you have any vacuum leaks, even ever so slight, those will also lean you out and make it difficult to achieve a steady idle.
Just my own opinion here... I WOULD NOT SEND A CARB to HOLLEY to have it rebuilt/refurbished the previous owner of my LT1 sent the carb to Holley and during the refurb the original 6808 jet block was rendered inoperable, warped and etched deep enough to get into passages. (FWIW Holley DID replace the 6808 with an incorrect 5271) Holley also used new bowl inlet valve lock screws without the bevel along with other newer fasteners.
I send my carbs to ERIC at Vintage Musclecar especially survivor carbs.

My 71 LT1 with the original never apart engine makes 14" vacuum at 900rpm idle with the valves set PER the DUKE /JOHNZ method .018 intake clearance and .021 Exhaust clearance.
Currently running nonethanol 87 with 25-30% SUNOCO 110 race to get somewhere around 95 octane, though I am planning on switching to straight 100LL when I run through the rest of my Sunoco 110.
I run as much initital advance as possible without the engine kicking back, a B26 vac can and the lightest springs on the distributor fly weights.
IT accelerates like a freight train to 5500-5700 which is as high as I typically let it rev.

This is at 750rpm idle.