1972 LT-1 Vacuum Whistle

[Dan A.]

I installed a 3999295 insulating carb gasket I got from Richard Fortier on my 72 LT-1 to avoid fuel percolation here in Texas. It seemed to accomplish that but since I installed it there is a vacuum whistle from the carb at idle. I have double checked all of the hoses and even pulled the carb back off and installed the thin gaskets between the intake manifold and the carb with the 3999295 sandwiched in between. No change. What am I missing here? Thanks in advance for the help.

[Bill B.]

Re: 1972 LT-1 Vacuum Whistle

If you take off your air cleaner assy and put your ear close to as much of the Carb as possible at idle, can you pin-point the area the whistle is coming from? Internal to the pri or secondary throats, or a specific location external to the throats?

[Dan A.]

[Dan A.]

Re: 1972 LT-1 Vacuum Whistle

Quick update - I the removed thick gasket and replaced it with a thin one and no more whistle. Looks like $58.00 down the drain. Has anyone else used one of these successfully?

[Patrick H.]

Re: 1972 LT-1 Vacuum Whistle

Quick update - I the removed thick gasket and replaced it with a thin one and no more whistle. Looks like $58.00 down the drain. Has anyone else used one of these successfully?

It would be interesting to compare the shape of them.
I suspect they are not exactly the same, and the difference would explain the whistle.

[Dan A.]

Re: 1972 LT-1 Vacuum Whistle

Patrick,

You are correct. The insulator style is 1/4” wide on the sides while the other one is 3/4”. Sorry the picture shows up twice - my first attempt at including pictures.

I 481D2BE7-43FF-44DF-86E2-C87B782C12D2.jpeg

[Patrick H.]

Re: 1972 LT-1 Vacuum Whistle

It would be interesting to see the smaller one overlayed on the larger, but I suspect that the extra-large hole is allowing a stream of air, thus the whistle.

[Dan A.]

Re: 1972 LT-1 Vacuum Whistle

Here you go. DD26882E-FFCC-44E4-B49C-040E2EBD3E9A.jpg

[Bill B.]

Re: 1972 LT-1 Vacuum Whistle

To help resolve fuel bowl evaporation issues on mine, I first consulted with Holley applications engineering and asked for recommendations after I checked hood clearance and determined that I could use a 1/2" spacer, but not a 1". Holley recommended a 4 hole spacer rather than the square cutout on my stock aluminum dual plane intake for the 4150 carb. I ordered the SUM-G1402 phenolic spacer from Summit Racing. It has the matching 1-11/16" hole diameters to match the throttle base plate throat dia. Installed and rechecked hood clearance at several places along the air cleaner lid with no issues. I noticed improvement in restart after cool-down, as previous shutdowns basically evaporated the fuel out of the bowls and would require several short crank cycles before it would restart after a long cool-down. After the spacer install, it would start on first crank cycle (almost immediately). Also noticed my hot idle was approx 50 rpm higher, and had to readjust back down. The Holley engineer indicated that the carb would benefit with better low rpm range response with the 4-hole spacer on a dual plane intake, while a single plane manifold would benefit the most with a square cut-out spacer.

[Dan A.]

Re: 1972 LT-1 Vacuum Whistle

Bill,

Thanks for the detailed information. I found a 1/4” phenolic spacer at Jomar Performance that is the same shape and size as the paper gasket and will let me use the original studs and fuel line configuration. We’ll see how it goes.