Mysterious Carburetor Problem
Thread Starter
Burning Brakes
Joined: Feb 2001
Posts: 929
Likes: 5
From: Marietta GA
Mysterious Carburetor Problem
I have a Holley 4011 (Spreadbore) 800 cfm double pump carb on my big block. The car has been hard to dial in because of carburetor issues, it won't idle consistently and often suddenly stalls with no warning.
It seems I have narrowed the problem but can't figure it out. I have properly set the float levels and there is no interference with the stock drop base air cleaner with any linkage etc. The car can be idling just fine but if I rotate the air cleaner a little bit right or left then gas will come out of the rear vent hole above the secondaries and kill the engine. I am told that those vent holes need a certain amount of clearance from the air cleaner base and I am using an adaptor which raises part of the air cleaner base above the vent hole. Any ideas. Is there a vacuum or siphon issue here, I have no idea. I promise there is no linkage other mechanical interference when I rotate the air cleaner. Thanks for any replies.
It seems I have narrowed the problem but can't figure it out. I have properly set the float levels and there is no interference with the stock drop base air cleaner with any linkage etc. The car can be idling just fine but if I rotate the air cleaner a little bit right or left then gas will come out of the rear vent hole above the secondaries and kill the engine. I am told that those vent holes need a certain amount of clearance from the air cleaner base and I am using an adaptor which raises part of the air cleaner base above the vent hole. Any ideas. Is there a vacuum or siphon issue here, I have no idea. I promise there is no linkage other mechanical interference when I rotate the air cleaner. Thanks for any replies.
Melting Slicks
Joined: Dec 2000
Posts: 2,041
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From: Wichita, KS
Re: Mysterious Carburetor Problem (BB68Vett)
Did you have your fingers crossed when you promised. If you did I'd say the accelarator pump was being pushed.
Try putting a piece of fuel line over the vents to raise the opening up a bit.
Try putting a piece of fuel line over the vents to raise the opening up a bit.
Le Mans Master
Joined: Mar 1999
Posts: 6,744
Likes: 1
From: delmont pa
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Re: Mysterious Carburetor Problem (BB68Vett)
for proper operation holley carb bowl vents needs 1/2" below the top cover of the air cleaner. if your vents are not cut on a angle cut them on a 45 degree angle for more area. :chevy
Tech Contributor
Joined: Aug 1999
Posts: 14,375
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From: At my Bar drinking and wrenching in Lafayette Colorado
Re: Mysterious Carburetor Problem (BB68Vett)
BB -
I've had the same experience as Clem: If your bowl vent tube is cut "flat" at the top, modify it by "scarfing" it back at a 45-degree angle. I'd do it to both the primary and the secondary tubes if I were you. Since the cutting & filing operation causes some metal debris, I prefer to do this by removing the carb and doing a partial disassembly.
I've had the same experience as Clem: If your bowl vent tube is cut "flat" at the top, modify it by "scarfing" it back at a 45-degree angle. I'd do it to both the primary and the secondary tubes if I were you. Since the cutting & filing operation causes some metal debris, I prefer to do this by removing the carb and doing a partial disassembly.
Thread Starter
Burning Brakes
Joined: Feb 2001
Posts: 929
Likes: 5
From: Marietta GA
Re: Mysterious Carburetor Problem (lars)
OK. Thanks. I, promise, promise, promise there are no linkage clearance/interference issues.
Both vent tubes were cut at 45 degrees from the factory.
Lars and Clem, I was told the same, namely that you need clearance below the vent tube for proper operation. The problem is that because I am using a stock drop base air cleaner, I need an adapter to make it fit on top of the Holley. I modified it to fit many years ago and this adaptor is about 1 inch high. So this means that the vent tubes are shrouded by the air cleaner base largely being above the level of the rear vent tube.
Now THE BIG QUESTIONS: What in the world does improper vent tube clearance have to do with proper carburetor operation? How does improper vent tube clearance cause fuel to come out of the vent tube and does the suggestion to add a piece of rubber hose to the vent tube to raise its relative level to the air cleaner base make sense in my instance and has it worked in the past?
As always, thanks for any replies. I just love being educated.
Both vent tubes were cut at 45 degrees from the factory.
Lars and Clem, I was told the same, namely that you need clearance below the vent tube for proper operation. The problem is that because I am using a stock drop base air cleaner, I need an adapter to make it fit on top of the Holley. I modified it to fit many years ago and this adaptor is about 1 inch high. So this means that the vent tubes are shrouded by the air cleaner base largely being above the level of the rear vent tube.
Now THE BIG QUESTIONS: What in the world does improper vent tube clearance have to do with proper carburetor operation? How does improper vent tube clearance cause fuel to come out of the vent tube and does the suggestion to add a piece of rubber hose to the vent tube to raise its relative level to the air cleaner base make sense in my instance and has it worked in the past?
As always, thanks for any replies. I just love being educated.
Le Mans Master
Joined: Mar 1999
Posts: 6,744
Likes: 1
From: delmont pa
Cruise-In I Veteran
Cruise-In II Veteran
Re: Mysterious Carburetor Problem (BB68Vett)
the fuel pump puts both fuel and pressure into the float bowl and the vents let the pressure out so the fuel is not forced out through the carb boosters. :chevy
Thread Starter
Burning Brakes
Joined: Feb 2001
Posts: 929
Likes: 5
From: Marietta GA
Re: Mysterious Carburetor Problem (clem zahrobsky)
So to summarize, since the vent tubes may not be able to freely vent the excess pressure, the fuel is forced through the boosters. Ok, so I understand that, but do you have any idea why this doesn't happen when the air cleaner base is perfectly centered on the carb but when rotated while the engine is running, this causes a problem.
