Holley 4175 vs AFB 3721
 

A friend just gave me a Holley 4175 that he used on his 68 327 (350 hp, I think). He only had the carb for 300 miles before replacing it with a new Rochester Q-Jet.

The current AFB 3721 (327/300 hp) on my 1963 is leaking. The SWC now has the small GM crate with a 327/350 cam and the original 327/340 intake and exhaust manifolds.

Would the Holley 4175 be a good substitute? What would be the advantages and disadvantages? Any replacement issues? Finally, it appears to have an electric choke. If so, can it be used?

My freind thought that the Holley was leaking, although there does not appear to be much/any staining (the carb looks brand new). If it was leaking and needs rebuilt, how does rebuilding a Holley compare to the AFB (which was easy). Thanks for your response.

Dick

FWIW:
I'd repair the AFB because I've had my belly full of Holly leaks, blown power valves etc.

Where is the AFB leaking?

I don't think than anyone will ever argue that an AFB is LESS reliable and prone to leak than a Holley. An AFB does not have any "wet" gaskets. Zip kits are readily available - just rebuilt it.

The 1963 Corvette Shop Manual has a complete section on AFB overhaul, but the 3721S/SA/SB were used from '64 to '65 and the 3721SB became the replacement for the 3461S used on '63 300HP/manual trans and 340 HP, and it has some slightly different specs.

Also, the 327/340 inlet manifold flange pattern is designed for the AFB. After '63 the 327 SHP inlet manifold was flanged for a Holley. Also, you won't have to Mickey Mouse the choke system, fuel lines, PCV, throttle linkage, etc.

It's a no-brainer!

Duke

Never heard of an AFB leaking. The gas bowl on an AFB is all in the bottom half. Nowhere does a gasket hold back the gas. Sounds like something is awfully wrong. You'd need a hole in the metal for the gas to leak.

If you change to the Holley, you'll need to change the fuel feed (no rubber hoses), the air cleaner, and the choke. That can all be done, but it's a pain and an expense.

And if it doesn't leak now, your Holley will leak, so you can do this all over again.

You may be able to adjust your Holley to get it to run right. Most people can. But a significant minority cannot. AFBs run right straight from the box.

Thanks for your responses.

P.S. Anyone need a Holley 4175?

Duke,

It leaks down the sides (i.e. between the top and bottom half of the carb). I sometimes get puddles in top of the manifold. Does that make sense?

Dick

MAYBE.......You Got Mail :thumbs:

Float level is a tad high.

It may be too high a float level, but if it's high enough to leak out the gasket the engine would probably be flooding.

Disassemble and clean everything. It could be just a tired gasket, but check the main body and bowl cover mating surfaces for flatness and dress them with a file to level, if necessary and don't overtighten the screws.

Be sure to carefully install the tiny O-ring between the choke housing and main body. Improper installation or omission of this O-ring will cause a vacuum leak and poor choke performance.

Very well worn AFBs can sometimes leak fuel out of the throttle shaft bores, which will cause stains and sometimes small puddles in the low spots of the inlet manifold. The situation is exacerbated by too high a float level which causes liquid fuel to drip out the nozzles after shutdown. The fuel then migrates down the throttle shafts and drips out the ends of the throttle shaft bores. To correct this (assuming the problem is not just too high a float level) the main body needs throttle shaft repair bushings, in which case the carb should be sent to a specialist.

Try to rebuild it yourself first and see what happens - cost and time for a basic cleaning and rebuild are low.

Duke

Duke and others,

Thanks and have a great New Year.

Dick

Again I must say, I have never, ever had a leaky Holley, and I've had a ton of them. [altho in this case I would recomend sticking with the carb that fits].