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I'm a new guy- got a 61 GMC 1/2 ton with the 305. looks like someone might have installed a later model carb since it appears to have an adapter on the manifold.2 bbl Holley -only numbers I see is 6R1919B cast on top of the air horn. looks like half a dual float Holley with a full size top about like a 4 BBL would have. so far unable to find a parts house that can find the numbers. the fuel pump diaphragm is dripping and I need to fix this thing. anyone dealt with this one? just bought this old truck and want to get it going again.. Raymond

Might be worth finding a cheap used one you've seen before, like a 600 or something. If not Holley has a site with all available carbs listed by part number.

If this unit is indeed a 2300 (looks like half of a 4150), then the number should be stamped (not cast) on the back side of the rectangular portion of the airhorn which houses the choke butterfly. You are looking for the word "LIST" followed by an "R-nnnn-???) where the nnnn is the part number. The ??? would refer to the engineering change level of the carb at the time of production.

An example:

LIST R-4412-AAS

Jon.

The original carb for a 1961 305 would have been a Stromberg, which (opinion, others will differ) is 10 times the carburetor compared to the Holley. They are also significantly more expensive, which is why they sometimes get changed by those looking at the immediate bottom line rather than the long term.

Jon.

well after further looking there is another set of numbers on the back side of the air horn. 74481 with 1674 below those. my CARQUEST guy could not pull them up though he thinks it may be some type high performance 2 BBL. he said he'd make some calls but at least he says he can get me a accelerator pump diaphragm for the base which is leaking and probably I better get a power valve also. I have been told they are bad to blow out as soon as one backfires with a Holley. I can't really tell if it is bad. most likely this is not an original carb since it has an adapter block bolted to the manifold- but I want it to run for now and likely I'll try to get a propane system to install on it if I drive it much. Raymond

Once it backfires they're toast. If through the intake of course.

Hey there Skelflyer...don't know if this would help, but when I was looking for help on my little 1bbl Rochester, I was referred to this guy by Patrick's out in Arizona. When I talked to the guy, he basically diagnosed my issue over the phone and by looking at about 3 pictures I emailed him...
KINGLONESTAR

carbsonly
BFIC Fuel Systems
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Burbank CA 91506
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The Holley parts are pretty standard and any local parts house should have a rack of them. Holley used two different accelerator diaphragms for the pumps, a 30cc and a 50cc. You can tell by looking, the casting under the pump is about 3/8" thick for the 50, and about 3/16" for 30cc. They will not interchange diaphragms, yet both are common parts.
Get a set of bowl gaskets,
http://www.holley.com/types/Gaskets.asp which you can compare yours to the new ones.

http://www.holley.com/types/Power%20Valves.asp Looking at this page from Holley, you can see there is a bunch of power valves available. Check what is in your carb before purchasing one. The number is the amount of vacuum it takes to open the valve, the small the number, the quicker they open. It works like metering rods, adding more fuel under acceleration.

And for the record, this is not the original carb! GM never used adapters.

Joe