Inliners International Forums General Discussion Engines 2bbl or 4bbl

Hi all, I posted this question recently on one of the truck boards with some good responses. Thought I'd ask for thoughts/ideas/opinions here as well while I mull this over:

The Suburban has a stock 292 except for an HEI distributor and a Rochester 2-bbl sitting on top of an Offenhauser manifold with a homemade adapter. The 2bbl was a huge improvement compared to the stock 1 bbl in terms of power and drivability. The carb is now prettty worn.

I'm deciding between rebuilding/replacing the 2 bbl vs a new small 4bbl carb (Holley 390, Edelbrock 500). My goal is driveability and MPG (weekend driver). Interested to hear thoughts and experiences with any of these options and why one option might be better than another. I keep going back and forth in my head about this...

The 4b can be better on fuel if you're not a lead foot driver lol. reason being smaller primary's But you could also go with a progressive Webber 2b carburetor.

I have a pretty much built 207 in my '53 Chevy pickup. I original built the engine in the late 70s. A couple of rear end ratios & 3 different transmissions. The best it ever ran was with a Rochester 2G. The best milage is with an Edelbrock 500 four bbl. Possibly a smaller 4 bbl (Holly 390) would be better. I have used every jet/ meter combo in the Edelbrock book and several that are not in it. For the 292 I have planned for my '68 flat bed I'm going with an early '90s GM TBI injection.

We need to be able to edit our posts!!!!!!! One of the stupid deficiencies of this site.

You can edit your own post just use the edit button in the lower right corner.

In the lower right corner of my own posts there are two choices. REPLY & QUOTE no EDIT On other people's posts there is a LIKE option.

You may have the EDIT option because you are a moderator.

I can see it on your post??? hummm?

Is there something you wanted to edit out or change??

You can see it because you have special powers. I had just noticed that in the post above I put 207 instead of 270 for my engine size. It doesn't matter except that is why I stopped posting about my projects here. Sometimes things need to be corrected and this forum doesn't allow that.

Now back to 292 Carry-all's Carb question.

You can edit a post up to 24 hours after you post it, then the edit button goes away. I think Titen did that because some people were going back and deleting their entire posts on a regular basis back when there was much more activity on here.

Proof read your post to make sure it�s correct. If not edit it before you post it.

I was going to use a 2g on my 292, even had an adapter made for my Aussie Speed intake, and then decided on a Sprint 250 Quadrajet. Had the sprint carb built at stage 1 to account for the 292 and the lumped head. If I do decide to use the 2g, I have that option.

I can't say how it runs yet, just got the block and head back from the machine shop yesterday. I now have to take inventory of my parts to see if I have everything to assemble it. It's been such a long wait I forgot what I bought...lol...

Well, it�s been a while - time for a summary/update. In deciding between a 2bbl and 4bbl, I came across an inexpensive, slightly damaged Rochester 4-jet (4GC). This carb hadn�t been on my radar, but I had one back in the day on a small V-8 and liked it, so thought, let�s give it a try. Long story short, I went through numerous times of disassembling and reassembling while I relearned carb rebuilding (it�s really a pretty simple carb) and chased down some fuel leaks. Meanwhile, I came across a manual choke version of the carb (4G) that still had the original id tag(!). The tag id�d the carb as period-correct for my truck - very exciting! So, rebuilt that carb and got the truck back on the road.

So far, the pros: truck starts much more easily when cold and part-throttle response is better than with the prior 2-jet. Cons: seat-of-the-pants power seems reduced (maybe not a big surprise given the smaller throttle bores) but MPG after the initial half-tank run was a little less than 13 MPG, about the same as with the 2-jet. I was hoping for at least 14 and wishing for 15. The spark plugs look pretty decent so far, so I think the jetting is about where it should be. I�ll keep driving as it is for now and see how things settle in over the longer run. Happy to answer any questions about the project and hear any further suggestions/advice.

I'm not familiar with that carb. What is the cfm rating? Are the secondary bores larger than the primaries? Are the secondaries mechanical or vacuum? The primaries are smaller than a 2 Jet?

The quadrajet had smaller primarys the seconardys about the size of a silver dollor LOL

Yeah, so this carb is prior to the quadrajet. Online info shows 3 different sized cfms available. The carbs I have have the same size primary and secondary bores, 1 1/4" if memory serves, and was rated at 486 cfm. The 2 jet bores were notably larger at 1 5/8". There are larger 4-jet carbs noted, these have slightly larger secondary bores and were presumably used on bigger cubic-inch engines such as Buick and Cadillac of the era.

The secondaries on these carbs are interesting - they are physically mechanical secondaries, but there's an additional set of spring-loaded butterfly valves that sit above the secondary throttle plates. They only allow flow through the secondaries if there is enough airflow to open those butterflies. The tension on the springs were factory set; there is a set screw that can alter that setting, but I know nothing about how that might be calibrated/adjusted.

Carbs get so involved so fast that I always listen to the experts. If what they have done is not exactly what I'm doing I will experiment. This sounds similar to a WCFB since primary & secondaries are thrsame size. My first 4bbl was and AFB on a warmed up Ford flathead sitting on an Edelbrock manifold. The most dependable street flathead I ever had. No BS linkage basically I just bolted it on and didn't try to understand it.

The secondaries on your carb sound like the beginning of sophistication on these things. My son & I built a 750 cfm Carter (Edelbrock) for his supercharged Studebaker V8. It ran mechanical secondaries to dump the extra fuel according to boost. I gets over my head pretty quick.

I know which one you're talking about now, I haven't seen one of those in a very very long time. Yes they were pretty much what they called the square bore carb Verse the spread bore carb. And if I recall right some of the early 283s that came with a 4b had it and I think it was also used on the dual intake 409s.

Yes, square bore, thanks! Apparently used on 327s as well (also seen references to the 348/409 engines as mentioned). So with the 292 six CID slotting in between the 283 and 327, figured this carb should be about right cfm/jetting-wise. I think the early Carter 4-bbls were square bore as well, but I have no exposure/experience with them.

The cfm rating on most modern carbs are calibrated for V8s. They don't flow the same on sixes but I can't remember the details of how it works. I'm pretty sure there is a thread here that explains this.

The 4 GC was a good carb. It did come in a early version with small bolt pattern ( bolt direct onto a Clifford/Offy intake with no adaptor) and larger bolt pattern.
I have a hundred Rochester rebuild kits for them.
Cubic in wise as a cylinder comparison. The 250 is similar to a 350 and a 292 like a 400.

Thanks Tom, I knew the sixes took more carb but didn't remember how much. That is quite a difference.

Hmm, the cfm difference in six vs. V-8 is new info for me. Can you guys explain it further? I know that 2-bbl vs 4-bbl are calculated differently for cfm, but I don't understand why the number of cylinders a carb is attached to should make a difference. MIght help explain why the 292 feels a bit underpowered with the 4-G compared to the 2-GC that was on it before (though I pulled that 2-CG directly off a junked 305 V-8 (similar CID to the 292)), so now I'm even more confused!

I did not mean that a 250 should take same carb as a 350. What I meant. 250/6=41.66 CU per cyl 350/8=43.75 CU

Ah, ok, that makes more sense to me now, thank you for clarifying!

I think what they were talking about was the methodology on sizing the cfm for a particular carb. 4bbl carbs are measured at 1.5 inches of Hg, and 3.0 for 2 bbl carbs. The quick and dirty is that a 2bbl carb rated at 500 cfm is 40% smaller then a 4bbl rated at 500 cfm.

So to convert a two-barrel rating into a four-barrel rating, divide the two-barrel rating by 1.414.
To convert the four-barrel rating to a two-barrel rating, multiply the four-barrel rating by 1.414.

For example, a 350 cfm Holley 2bbl flows 247.52. So to get the same flow as a four bbl 500 cfm, you would need two 350 carbs, or a 700 cfm two bbl, lol.

That 292 is just begging for a Rochester Quadrajet calibrated for a Chevy 305 or an Olds 307 engine. It has tiny primary barrels with a 3-step venturi, and a secondary air valve that prevents overcarburetion when those huge secondary throttle plates open up. The "small" Q-Jet has a 750 CFM rating, yet there are no flat spots from idle to all the CFM the engine can flow. I ran a 460 cubic inch truck pull engine based on a MOPAR 440, running a QJ for a 455 Buick. It dynoed at 714 HP at 6500 RPM on one carb and aviation gasoline. The carb would let it wind to 7500 RPM or more, but the torque started dropping off at that speed.
Jerry

I knew a few guys who ran them on the 250 once they are adjust right they would run like a bandit.