Vacuum Leak

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Jul 28, 2015 | 02:01 PM

bb_66

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From: Milwaukie Oregon

Vacuum Leak

I'm having trouble tracking down a vacuum leak from what appears to be coming form inside the carburetor. After the car is warmed up there is a high pitch leak from the under the choke lid (inside the carburetor). Could this be a bad carburetor to intake manifold gasket? 427/390hp stock Holley carb

Jul 28, 2015 | 03:22 PM

MelWff

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Have you tried to gently tighten the 8 fuel bowl screws?
have you tried to gently tighten the 4 carburetor base nuts?
Do any tightening on a completely cold engine.

Jul 28, 2015 | 03:50 PM

bb_66

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From: Milwaukie Oregon

The carburetor base nuts were tight. I'll check the fuel bowl screws tonight. It sounds like a fairly large leak but am unable to pinpoint it from outside the carburetor.

Jul 28, 2015 | 04:01 PM

JohnZ

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From: Washington Michigan

Quote:

Originally Posted by bb_66

The carburetor base nuts were tight. I'll check the fuel bowl screws tonight. It sounds like a fairly large leak but am unable to pinpoint it from outside the carburetor.

That "classic" Holley vacuum leak whistling sound is usually the result of a bad baseplate-to-main body gasket or baseplate-to-main body screws that aren't tight.

Jul 28, 2015 | 04:19 PM

bb_66

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From: Milwaukie Oregon

Okay, that's what I expected it may be. It looks like there are two gaskets one metal and the other a compressed ceramic fiber like material. Is this correct? -- thanks guys

Jul 28, 2015 | 04:21 PM

jedjr59restomd

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From: South Jordan Utah

You could try a little carb cleaner spray with the red nozzle tube on it, and watch/listen for an increase in your RPMs Just takes a little bit at a time directed at the suspected area. Usually works for me if external....
Best of luck!

Jul 28, 2015 | 04:36 PM

JohnZ

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From: Washington Michigan

Quote:

Originally Posted by bb_66

Okay, that's what I expected it may be. It looks like there are two gaskets one metal and the other a compressed ceramic fiber like material. Is this correct? -- thanks guys

Those are the gaskets BELOW the carburetor, on the intake manifold - the order is manifold, gasket, stainless baffle, carburetor. The phillips screws you want to check are in the bottom of the baseplate, after you remove the carburetor.

Last edited by JohnZ; Jul 30, 2015 at 03:10 PM. Reason: Revised order of gasket and baffle.

Jul 28, 2015 | 04:39 PM

bb_66

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From: Milwaukie Oregon

Ah, okay thanks for the clarification John. - mike

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Jul 28, 2015 | 06:17 PM

Easy Rhino

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From: Middle TN by way of KY, OH, VA, IL, CA, FL, NY, SC, HI

a sure-fire way to locate a vacuum leak is to use an UNLIT propane torch with the engine running and the UNLIT propane torch with the valve open, passing the torch tip slowly around the intake manifold and carburetor fittings, hoses, and interfaces.

When the propane finds a vacuum leak, it gets sucked into the intake, and the engine will speed up.

I have been using this technique for decades, with pin point and immediate results.

You can also use an automotive stethoscope, just pass the tip over possible leak locations and listen for the loudest noise site.

In either case, be safe.

Jul 30, 2015 | 01:03 PM

#10

bb_66

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From: Milwaukie Oregon

I removed the carburetor last night and a few of the base plate screws were loose. After tightening them up and reinstalling the carb no more whistle.

John, you mentioned the order for the stainless and paper gasket are to be installed. I've attached pics but not sure I have the privileges to submit them.

The intake manifold has a slot in the front that is covered up by the stainless gasket. Is this correct? I noticed the base gasket I picked up also has the slot cut out. Here is way my gaskets were installed after removing the carburetor: Manifold, paper gasket (with cutout), stainless gasket, paper gasket without cutout, carburetor. -- can I send you the pics via pm?

Attached Images

Jul 30, 2015 | 01:37 PM

#11

DansYellow66

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From: Central Arkansas

I'm kind of confused by the order of pictures in your last post and description of gaskets - but I do have a suggestion. I would not use the single opening gasket on that intake. I once had a Carter AFB on my GTX with an open gasket like that and it whistled like mad. After replacing it with a proper 4-hole gasket the whistling stopped.

Jul 30, 2015 | 02:28 PM

#12

20round

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From: montague michigan

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Quote:

Originally Posted by bb_66

The carburetor base nuts were tight. I'll check the fuel bowl screws tonight. It sounds like a fairly large leak but am unable to pinpoint it from outside the carburetor.

i have run into this alot over the years.......... check your throttle shaft for play .. any play in this area will create havic in a holley.. if i rememeber, those 4160 holleys have a passage in the center by the front throttle shaft that gets burnt and wore... a little carb cleaner or wd-40 sprayed around will find it fast.. i hold up a peice of card board around the carb throut so that you dont get a false idle gain..good luck..troy

Jul 30, 2015 | 02:54 PM

#13

bb_66

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From: Milwaukie Oregon

This was the ordering when i removed the carburetor.

Carb
|
Paper gasket (large cutout)
|
Stainless gasket
|
Paper gasket (similar to pic below)
|
Intake Manifold

Any idea what the slot is or does on the intake manifold? The gasket specified for my carburetor (3370 Holley) -- see pic below, has the slot cut into the gasket allowing the slot to be exposed to the carburetor base plate. However, if I use the stainless gasket it will cover up the intake manifold slot.

I should note the motor runs fine, much better after tightening the base plate screws. I'm just a little perplexed whether this slot should be exposed to the base plate.

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Jul 30, 2015 | 03:23 PM

#14

JohnZ

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From: Washington Michigan

Quote:

Originally Posted by bb_66

I revised the incorrect order of the parts in my original post - the order you noted on disassembly is correct, except for the extra one-hole gasket between the stainless baffle and the carburetor; there's only ONE gasket and ONE stainless baffle.

The stainless baffle WILL cover up that slot, to protect the carburetor baseplate from direct impingement of hot exhaust gases which will eventually warp or crack the baseplate; even with the baffle, the heat from that slot will eventually damage the carburetor.

We call that slot the "hot-slot", and the holes at the ends should be plugged, either with 7/16" steel cup plugs, or by tapping the holes and installing 1/4" NPT allen-head plugs.

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Jul 30, 2015 | 03:47 PM

#15

bb_66

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From: Milwaukie Oregon

Is there much pressure build up coming from the 7/16" holes? Are these steel cup plugs something I can order? -- thanks, mike

Jul 30, 2015 | 04:08 PM

#16

JohnZ

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From: Washington Michigan

Quote:

Originally Posted by bb_66

Is there much pressure build up coming from the 7/16" holes? Are these steel cup plugs something I can order? -- thanks, mike

Those two holes connect to the crossover passage below the floor of the intake plenum, so they're carrying exhaust pressure and heat; many auto parts stores carry Dorman cup plugs, and if the cast holes are too big for the cup plugs, any speed shop (or Jeg's or Summit) will have the allen-head pipe plugs. If you use the advanced search function with "hot-slot" as the key word, you'll find LOTS of info on it.

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