4150 Holley - '72 LT1

[Jerome P.]

The 4150 Holley on my '72 continues to leak a small amount of gas. I have replaced the accelerator pump and gasket and tighten the 4 screws. I have also tighten the 4 bolts on both the primary and secondary bowls. The fuel input lines at both the primary and secondary bowls have also been tighten. This corrects the leak problem for a time, but it does return.

I have heard the 4150's do occasionally leak gas. Is this true? One way or another I would like to get to the bottom of this problem. Any thoughts, comments or suggestions will be appreciated.

[Paul O.]

Re: 4150 Holley - '72 LT1

Jerome

On my old 71 LT-1 leaks tended to be at the fuel line to the inlet bowl area. Not to say it could not be leaking at other places but that was were most of my leaks occurred. It seemed that the rigidity of the "Y" block and metal lines along with engine vibration would cause the fittings to loosen over time. In some cases the bowl threads would then be damaged after several years from this issue.

[Terry M.]

Re: 4150 Holley - '72 LT1

The fuel line support to the rear passenger side carburetor attaching stud adds a lot of rigidity to the fuel line assembly. Chevrolet didn't design and install that support for nothing.

[Kevin G.]

Re: 4150 Holley - '72 LT1

Hi Jerome,

If I allow my car to sit for a period long enough for the gaskets to dry, the Holley carb will leak at the bowls. Every time I do start it up I have to tighten the eight screws on each side. Shortly after running the gasket swell and leaks dry up. I've learned to live with it......

[Steve G.]

Re: 4150 Holley - '72 LT1

Hi Jerome,

If I allow my car to sit for a period long enough for the gaskets to dry, the Holley carb will leak at the bowls. Every time I do start it up I have to tighten the eight screws on each side. Shortly after running the gasket swell and leaks dry up. I've learned to live with it......

Kevin,
This is what causes the metering blocks to distort and leak internally, the drip down the throat from the idle orifices. This and removing the bowls and re-installing without replacing the gasket between the metering block and body.

The gasket between the block and the body is a fairly thick cork gasket that runs across the entire face, unlike the bowl gasket that just goes around the perimeter. There are two screws near the centre of the block that fasten it to the body. The outside of the block is clamped to the body by the bowl screws. When you tighten the bowl screws without tightening the centre ones you compress only the perimeter of the block to body gasket. This causes the block to distort to a concave shape and allows fuel to cross passages without dripping to the exterior. Not at all uncommon with Holley carbs.

In the perfect world, every time any of those 12 screws were tightened it would be with new body to block and block to bowl gaskets and all screws torqued evenly and to spec. That's not where we live.

Apparently Holley offers a gasket set that deals with this issue. They are reuseable for those that like to remove their bowls periodically and I presume made of a less compressible material to avoid the distortion. Apparently replacement metering blocks are also reinforced to resist distortion. The synthetic material may also aid with the issue of leaking after sitting. It may be worth looking into.

Steve

[Steven B.]

Re: 4150 Holley - '72 LT1

I've seen distortion with holley carburetors with a plate and thin block on the secondary side, but not on the thicker blocks with adjusting screws. Is it possible for a block to distort? It would seem to me that the block would have lesser issues with distortion.
Steve

[Steve G.]

Re: 4150 Holley - '72 LT1

I've seen distortion with holley carburetors with a plate and thin block on the secondary side, but not on the thicker blocks with adjusting screws. Is it possible for a block to distort? It would seem to me that the block would have lesser issues with distortion.
Steve

Steve,
It's actually the front one with the idle circuit where the issue is usually caught. It's the cross leakage which makes it's way to the lower than fuel level idle circuit entry into the throat that gets noticed. There's no idle circuit in the rear block so the warpage doesn't manifest as an operating problem. Those appear to be caught on overhaul.

I'm sure you are right that it takes more to warp the thick one, but it does get done on a regular basis.
Steve

[Timothy B.]

Re: 4150 Holley - '72 LT1

Steve,

On a Holley carburetor there is a idle circuit on the secondary side weather the carburetor has a secondary metering plate or metering body.

I have no idea what two screws you are talking about tightening to secure the metering body only the four bowl screws.

[Jerome P.]

[Steve G.]

Re: 4150 Holley - '72 LT1

Steve,

On a Holley carburetor there is a idle circuit on the secondary side weather the carburetor has a secondary metering plate or metering body.

I have no idea what two screws you are talking about tightening to secure the metering body only the four bowl screws.

The two clutch head screws, Only on the secondary side if I remember correctly. Are you saying that there is are idle fuel and transitions slots in the throat on the secondary side? With no mixture adjusting screws and the throttle plate closed up?

Steve

[Timothy B.]

Re: 4150 Holley - '72 LT1

Steve,

Clutch head screws hold the secondary metering plate on a 4160 Holley. No such screw used on the primary metering block or carbs with a secondary metering block (4150 Holley).

Yes, there are idle fuel and transfer slots on vacuum secondary carburetor. No mixture adjusting screws simply means it's not a four corner adjustment carburetor. Vacuum secondary carburetors have rich idle circuits because no secondary accelerator pump to get it started. With only a two corner adjustment carburetor there are little constant feed holes below the secondary transfer slots to keep the fuel fresh and the slot primed.

[Steve G.]

Re: 4150 Holley - '72 LT1

Steve,

Clutch head screws hold the secondary metering plate on a 4160 Holley. No such screw used on the primary metering block or carbs with a secondary metering block (4150 Holley).

Yes, there are idle fuel and transfer slots on vacuum secondary carburetor. No mixture adjusting screws simply means it's not a four corner adjustment carburetor. Vacuum secondary carburetors have rich idle circuits because no secondary accelerator pump to get it started. With only a two corner adjustment carburetor there are little constant feed holes below the secondary transfer slots to keep the fuel fresh and the slot primed.

Tim,

Clearly my recollection or familiarity of the various Holley carbs isn't as good as yours. But I was recalling those clutch head screws between the bowl and the body somewhere. Either way, whether there are screws in the centre or not, continual retightening and over tightening of the bowl screws, along with re-using gaskets is what distorts the blocks and plates.

I wasn't really thinking of the orifices in the secondary as idle circuit, because they aren't in use at idle. As far as I know, the throttle plates are closed and there is no air or fuel flow at idle.

But I do stand corrected in that they can drip fuel with a distorted plate.

Steve

[Steve G.]

Re: 4150 Holley - '72 LT1

Every carb can leak fuel. They can leak anywhere there is fuel and a joint. Even sometimes when there is no joint, as in a crack. There is no one place to point on any model of carb to say that this is the only place they ever leak.

Retightening and overtightening the bowl screws will distort the block and lead to more and other issues. Over tightening anything runs the risk of damage, stripped threads, cracked parts, rounded heads ....

In my shop the way we deal with a fuel leak on any part anywhere is to blow the entire area dry and start it up and watch where the sweating/seeping starts and positively identify the source. I keep my air hose in my hand and I keep blowing away the fuel until I clearly see it appear out whatever the compromised point is. Sometimes I have to get my little inspection mirror out.

Unless the fault is something obviously loose, I take whatever it is apart and examine it to identify the issue. Repair as required. Always reassemble with new gaskets.

Even if found obviously loose, if a gasket is involved, we replace it. Gaskets are, unless specifically stated, single use items.

There is no reason to live with constantly retightening the bowl screws to deal with a persistent leak. Repaired properly and left alone it will last a very very long time. There is a 67 Chrysler with a Holley in our yard that has been sitting for about 20 years. It gets started every few years to be moved and it has, as yet, never leaked fuel from the bowls. There's no reason yours can't as well.

Steve

[Kevin G.]

Re: 4150 Holley - '72 LT1

Kevin,
This is what causes the metering blocks to distort and leak internally, the drip down the throat from the idle orifices. This and removing the bowls and re-installing without replacing the gasket between the metering block and body.

The gasket between the block and the body is a fairly thick cork gasket that runs across the entire face, unlike the bowl gasket that just goes around the perimeter. There are two screws near the centre of the block that fasten it to the body. The outside of the block is clamped to the body by the bowl screws. When you tighten the bowl screws without tightening the centre ones you compress only the perimeter of the block to body gasket. This causes the block to distort to a concave shape and allows fuel to cross passages without dripping to the exterior. Not at all uncommon with Holley carbs.

In the perfect world, every time any of those 12 screws were tightened it would be with new body to block and block to bowl gaskets and all screws torqued evenly and to spec. That's not where we live.

Apparently Holley offers a gasket set that deals with this issue. They are reuseable for those that like to remove their bowls periodically and I presume made of a less compressible material to avoid the distortion. Apparently replacement metering blocks are also reinforced to resist distortion. The synthetic material may also aid with the issue of leaking after sitting. It may be worth looking into.

Steve

Steve,

Thanks for the education! I'll look into the gaskets from Holley. Lets hope I haven't distorted any parts! This is the first car I own equipt with a Holley......