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hole in carburetor throat?
https://slantsix.org/forum/viewtopic.php?t=9064		 Page 1 of 1
Author:  steponmebbbboom [ Fri Apr 16, 2004 4:25 pm ]
Post subject:  hole in carburetor throat?
I noticed when assembling my Carter BBS there was a hole going from the base of the carburetor throat to the outside. I threaded it and installed a setscrew, thinking a lead plug had been knocked out.

However, when I removed my 1920 to install the BBS, I noticed a hole in roughly the middle of the barrel, drilled diagonally and pointing upward! It appears to be original. Why do these carburetors have unfiltered air being allowed into the carburetor throat? And could this have anything to do with my BBS running lean, even with the mixture screw bottomed? It was rebuilt to specs, the parts were clean and it wouldnt run the engine unless I closed the choke plate to within 1/4" of the edge of the barrel. (My 1920 also ran slightly lean. I couldnt enrich the mixture regardless of the mixture setting. My first instincts are a leaking intake gasket...)

So what's the purpose of this hole? Can I filter it? Is it there to relieve positive pressure from turbulence, perhaps?
Author:  mpgFanatic [ Fri Apr 16, 2004 5:33 pm ]
Post subject:  Re: hole in carburetor throat?
Quote:
However, when I removed my 1920 to install the BBS, I noticed a hole in roughly the middle of the barrel, drilled diagonally and pointing upward! It appears to be original. Why do these carburetors have unfiltered air being allowed into the carburetor throat?
I saw exactly the same thing in a 1920 I got from Charlie, asked him the same question, and pretty much accused him of putting it there... he said he hadn't used the 1Bbl carb since 1975, and he had no idea why he would have drilled a hole there, except maybe to sneak something past the rulebook...

Perhaps he wasn't trying to cheat, after all. ;)

- Erik
Author:  Slant Cecil [ Fri Apr 16, 2004 5:44 pm ]
Post subject: 
Sounds like your describing a venturi vacuum port.

Cecil
Author:  mpgFanatic [ Fri Apr 16, 2004 5:57 pm ]
Post subject:  no nipples. :)
Quote:
Sounds like your describing a venturi vacuum port.
Well, there's no hose nipple... it looks exactly like someone attacked the center of the main body with a 1/16 drill bit. I'll post a pic later if you want, no camera with me right now.

- Erik
Author:  HyperValiant [ Fri Apr 16, 2004 6:53 pm ]
Post subject: 
I have seen this on three different 1920 holleys and wondered the same thing.
HyperValiant
Author:  '68signet [ Fri Apr 16, 2004 8:05 pm ]
Post subject: 
My C.A.P.(cleaner air package) 225 equipped 68 Val had exactly the same hole in the side of it's holley 1920. It's a previous "little old lady" owned car so I'm pretty sure it's all original equipment.

I think the hole has to do with the C.A.P. equipment (carb and distributor)to help lean up the carb mixture to reduce emissions or some crap like that. It is a factory modification, although a poor one.

I replaced the Holley with a C.A.P. BBS and it also had the same hole in the side so I plugged it with a small brass plug and it runs fine.
Author:  Marc [ Fri Apr 16, 2004 8:07 pm ]
Post subject: 
I was pondering that very question today. I have two Carter BBS's and they both have that hole. The BBD on my '68 318 also has a hole into each barrel.
Author:  Doctor Dodge [ Fri Apr 16, 2004 9:27 pm ]
Post subject: 
The only thing I have noticed with this hole is that it has an effect on the ported vacuum signal and when the vacuum advance "tips-in".
Plug this hole, the vacuum advance comes-in sooner.
These days I don't mess with this hole, if I need to adjust when the vacuum advance kicks-in, I install an adjustable rate vacuum advance pod.
DD
Author:  SlantSixDan [ Sat Apr 17, 2004 2:51 pm ]
Post subject: 
Doug's right. The hole is there (from the factory) to smooth the initial stages of vacuum advance *and* power valve activation. Some carbs have the hole, other calibrations of the same model carb do not. I've found it best to leave it as is (if the carb had the hole, leave it. If it didn't have the hole...leave it!)
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