Inliners International Forums General Discussion Engines Carter 2bbl on Offy 4bbl intake

Recently, I've been running a 470 Holly Truck Avenger carb on top of an Offenhauser intake in my daily driven '71 Chevy C20 w/ 250ci and Muncie 4-speed (with the granny gear, so technically its like a 3-speed). At full throttle, the 4bbl's open, and the rpm's don't take off like they should - yeah, I overshot the carb for the stock engine.
I happened to pick up a Carter 2bbl (#4877S) at a local swap meet, and thought I'd pop this in place of the 470. What I could find about the carb is that its from a 61-64 Olds 394ci. I realize cfm ratings weren't posted until the early 70's, so here are my questions:
1. Any tech information about this carb? Book, website?
2. Ever try a 2bbl on a 4bbl Offy intake? (I'll be machining the baseplate instead of using a 2bbl adapter on top of the 4bbl Offy baseplate)
Thanks!

Years ago I had a '67 SW chevy pickup w/ 250 stocker, only mod was hedmans/flowmasters. I used the old common 585-600cfm holley w/ vacum secondaries - worked fine. Also used a holley "Economaster" ( an economy replacement for a Q-jet) I believe it had mech. secondaries- it was only 450 cfm. it was for a 350 chevy Q-jet replacement, but I found the same basic carb was used with some mods as a buick V6 replacement -I approximated those changes (mostly idle circuit ports) to mine and it was a fine street carb. Point being, dont sell your 6's carb handling capabilities short. I would rather have a progressive 4 bbl. than a full time 2 bbl. The large 2GC rodchesters that would have been on those Oldsmobiles had 1-11/16 throttle bores and 1-3/16 venturis(in the mid 250 cfm range in four bbl talk). Try to find Doug Roe's Rodchester carb book-it tells all about almost all rochesters from BC one bbls to Electronic dual jets- HP is the publisher, he cfms a lot of rochesters. Two bbl cfm's need to be multipled by approx. .7 to equate to standard 4 bbl talk-thus the old holley 2bbl 500 is only about 350cfm when compaired to a 500 cfm 4 bbl. By the way, my old 250 would'nt rev much past 5000 in neutral, off a cliff! But the 4 bbls did made a difference-would'nt trade them for a stock one bbl or a two bbl for anything.

Is the exhaust system stock or ?

Yeah, stock single exhaust. I'm planning on running sidepipes soon (single 2" to 'y' pipe with dual 2"). Would I be better off changing out the jets in the 470 to something smaller? Any ideas on sizes? I'm not getting into the engine, and I don't plan on running headers because I don't want to deal with adding water ports to the intake or running the coolant lines; seems more of a hassle than what it's worth on a daily driver.

You could always install the 292 truck manifold that has the larger exit for better exhaust flow.

To see where you A/F mixture is at,need to install a set of new plugs & read the plugs.

I would use a wide band O2 reader to be more accurate & takes out the guesswork or you or anyone for that matter misreading the plugs.
Two cents thrown.


MBHD

As edski reflected, that Olds 2bbl will have two kinda big primary venturis. The primary of your Holley is only a 1-3/32 venturi, about 220cfm or so from the primaries only - smaller than that Olds 2bbl. So if the 4bbl is too much for the stock 250 (I have the same situation) maybe just block the secondary SHUT on the Holley and you have a "free" 2bbl with no adapters, no hassle, and you're ready when you need the extra airflow.

All the jetting I can find on the 450-465cfm Holley ranges from #50 to #53 in the primaries so you might see what you have and base it on that.

About the only reason to go to a 2bbl would be if it was a progressive Pinto/Weber style 5200 carb. They flow about 203cfm total (4bbl style rating at 1.5" Hg)and are a progressive 2bbl so you cruise on the tiny primary 1bbl. Great carb for response and mileage. A tad small for even your stock 250 but then again the 470 Holley is quite a bit too big.

What you should be careful of if you block the Holley secondaries is that it confines all vacuum to the primaries.
If your WOT vacuum is high enough, your power valve may close at full power - not good.
The air/fuel curve will also go richer than you want, but IDK how much.

Thanks everyone!