2-bbl or 4bbl for 292?

[67carry-all]

Hi all - would greatly appreciate your informed opinions and especially experience with this situation. Background: a number of years ago, I replaced the much-despised factory 1-bbl on my ’67 292 with a junkyard Rochester 2-bbl from a 305 V-8. The 2-bbl sits on a homemade adapter on top of an Offenhauser 4-bbl manifold. The improvement with the 2-bbl was tremendous - the big six felt like it could finally breathe, there was more power, and drivability was substantially improved. MPG stayed about the same around 13.

The old 2-bbl is now fairly worn and needs replacement. I’m deciding between replacing it with a similar rebuilt unit vs. a new small 4-bbl such as a Holley 390cfm. The truck is a weekend driver and I’m more concerned with drivability and MPG than power. The motor has an HEI and is otherwise stock. So, what do you all think - 2 or 4bbl?

[mattfranklin]

It depends on what is important to your decision.
I think you're probably reaching the point of diminishing returns on airflow capacity.
The motor can't breathe more not matter how big the carb is.
What the bigger carb did do for you was decrease the pressure drop from atmospheric.
If the pressure drop from atmospheric is low, your MAP (or manifold absolute pressure) can't increase.
EngineMasters had a nice episode on carb sizing.
And the Rochester is probably cheap and simple.
There is an elegance and coolness to a clean simple 2bbl.

However, depending on what you like and your budget, Holley is flashier and could have more of a coolness factor. Totally personal taste. There is also more information out there on tuning and tinkering and optimizing Holley carbs. The 390 cfm is probably a good range to be responsive when you accelerate.

Good luck in your decision.

[kwmech]

The 2bbl is enough of a carb for the 292 unless you have future plans to hot rod it

[mongocanfly]

I ran a 390 Holley on my cammed 250 6cyl yrs ago.. did very well..

[scottofksu]

Just to play devils advocate... You may consider going with the small 4 barrel as the primaries that you will spend the vast majority of your time running on might make it act like a smaller carb from a MPG perspective than your 2 barrel - depending on the size of your 2 barrel. However, you will still have the secondaries to open if you want to have a little fun! The key will be the size of your Rochester. As I understand it they were sized based on application and came in 278-435 CFM. If your Rochester is the same CFM as the Holley 4bbl you are interested in (i.e., 390 CFM) that would make the primaries of the Holley 1/2 the size of the Rochester as the CFM ratings contemplates both the primaries and the secondaries for the 4bbl. Just some food for thought!

[scottofksu]

Note, to directly compare the advertised cubic feet per minute (CFM) of your 2bbl to your 4bbl you have to multiply the 2bbl CFM by .707 given differences in how they arrive at the ratings (e.g., 2bbls are tested at 3 inches of vacuum and 4bbls are tested at 1.5 inches).

[67carry-all]

Hi all, thanks for the great replies! I’m reminded of some of the reasons I initially went with the 2-bbl. In addition to being inexpensive, it does have a certain cool elegance, in my eye an almost factory look being a Rochester, and right down to the mechanical throttle linkage. I also like the idea of a 4-bbl sitting atop the Offenhauser and the possibility of even crisper throttle response with the small primaries. I’ll chew on this for a bit more and post an update when the new or newer carb is on the truck. Thanks also for the notes on cam.

['68OrangeSunshine]

Originally Posted by 67carry-all

Hi all - would greatly appreciate your informed opinions and especially experience with this situation. Background: a number of years ago, I replaced the much-despised factory 1-bbl on my ’67 292 with a junkyard Rochester 2-bbl from a 305 V-8. The 2-bbl sits on a homemade adapter on top of an Offenhauser 4-bbl manifold. The improvement with the 2-bbl was tremendous - the big six felt like it could finally breathe, there was more power, and drivability was substantially improved. MPG stayed about the same around 13.

The old 2-bbl is now fairly worn and needs replacement. I’m deciding between replacing it with a similar rebuilt unit vs. a new small 4-bbl such as a Holley 390cfm. The truck is a weekend driver and I’m more concerned with drivability and MPG than power. The motor has an HEI and is otherwise stock. So, what do you all think - 2 or 4bbl?

I have the Offenhauser intake [5716] and Clifford headers. '68 C/10 Stepside. 292 L6, Rebuilt original block @ .030-over, Crane 260H cam. HEI.
Currently [since '05] I run an Edelbrock 500 CFM 1404 4Bbl.
I used to run a Holley R8007 390 CFM 4 Bbl, but it got unstable after 25 years. To make it fit, I had to saw off the kick-down lever from the carb linkage. Not a problem since I have an SM465 4-Speed. I didn't like the Holley's tendency to blow a Power Valve at every big backfire. They may have fixed that, IDK. I moved on from the Holley.
I have great response with the Edelbrock [which is a copy of the Carter AFB = aluminum four barrel]. Easy to tune and get parts for.
I don't know of any good 2 barrels.
Is 500 CFM too much carburetor for a 292 @ .030-over? [296 actual CuIn].
Well, according to the charts, yes, but the vacuum secondaries only give as much Fuel/Air as the engine can process, so no.

You also might want to look into converting to a cable-actuated Accelerator/Throttle set up. The '67/'68 threaded rod has limitations.
Mine had a bellcrank -- Rube Goldberg deal -- and it would jam, freeze up, drop critical parts -- at the worst time. A cable throttle solved all that.

[67carry-all]

Thanks for that input, '68OS. I was somewhat leaning toward replacing the 2-bbl with a similar one, but you've given me some more to think about with the Edelbrock info - appreciate it.

['68OrangeSunshine]

Have you been to Inliners International?
www.inliners.org
I haven't been on there in a while, but they are Inline oriented.
While Chevy L6s are well represented, it is not a brand-specific site..

[67carry-all]

Yes, thank you for the reminder. It's one of those sites I forget about and then rediscover from time to time!

[AcampoDave]

Originally Posted by 67carry-all

Thanks for that input, '68OS. I was somewhat leaning toward replacing the 2-bbl with a similar one, but you've given me some more to think about with the Edelbrock info - appreciate it.

Here's some more info on the Eddy 500CFM from my personal experience on a 283. https://67-72chevytrucks.com/vboard/....php?p=9129708

[67carry-all]

Thanks for that, enjoyed the write-up. The AFM looks like a great tool to have, probably can't justify it at my end, though. Interesting that the Rochester 2-bbl seemed to be right on spec. The mulling over continues.

[bnorth]

This is an interesting watch and leads me to think the 2bbl is well matched to a mostly stock 292.
https://www.youtube.com/watch?v=e9-oJDlskQs

[67carry-all]

1) It’s been a while since I first posted about deciding 2bbl vs 4bbl, thought it was time to update the yet on-going saga. While I was deciding on which way to go, I came across a CL ad for an old Rochester 4-jet. I had one years ago on a small V-8 and remember it working well. On-line reference put the cfm of the smaller 4-jets at 486, should be good for a 292. Plus, asking price was $25(!). So, decision made. I picked up the carb, which had had some abuse in it’s long life, but nothing unfixable. Some cleaning and a carb kit later, it was ready to go.

My Offenhasuer intake is an older version with small, nearly square mounting holes. An adapter would need to be fabbed. I sketched out my idea on a piece of paper, my son put it into the computer and printed out a 3-D plastic model. Adjustments in hole placements were fine-tuned, then the data was sent out to an on-line metal laser-cutting service. A few days later, the part was in-hand...

[67carry-all]

2) The Suburban originally had a mechanical linkage from the accelerator pedal to the carb. Though I understand it’s not everyone’s choice, I wanted to keep it that way. Turns out that early/mid 80’s Mercedes (I forget if gas or diesel version) have crazy amounts of mechanical linkages on their engines. I had pulled a variety of pushrods, bellcranks, ball/socket joints, and brackets from some wrecking yard engines years ago for my original 2-bbl conversion. Sorting through, I was able to find a working combo of parts to control the 4-jet. The carb adapter was designed with these in mind.

So, got everything hooked up and fired up the motor. Fired and idled well, then started pouring gasoline from all over the carb. It’s been a while since I’ve rebuilt a carb and apparently I didn’t have the float needle clips installed properly. Took the carb off, pulled the top, removed the clips, reassembled, reinstalled, and fired up again. Again, ran great, but instead of flooding everywhere, seemed gas was coming out near the accelerator pump boot. Strange. Disassembled, decided to lower the float level, reassembled, same problem. I ended up doing multiple versions of this including swapping out floats from another carb I’d come across, but the “flooding” persisted, could not get it figured out. This all took place over a matter of weeks...

[67carry-all]

3) Meanwhile, I’d also come across a manual choke version of the carb that still had the identifying carb tag on it, correct for mid-60’s truck with a 327. Perfect! Got that carb, it was really filthy on the outside, decent on the inside. Got the outside mostly cleaned, but noticed my various cleaning fluids were stripping off the gold anodizing. Wanting to try to keep what I could of that, the outside of the carb is much cleaner but not perfect. Floats were again set lower than specs to (in my mind) minimize the chance of flooding (a bit traumatized at this point!).

Carbs were swapped on the engine, engine fired, ran and idled well, then darn it all if there wasn’t some gas leaking in about the same area as the other carb. A closer look, though, showed there was actually a leak from the connection of the fuel line adapter to the inlet. An internal gasket solved the leak. In hindsight so far, that was likely the issue with the continued “flooding” of the original 4-jet. The fuel inlet casting on that carb is designed a little differently, so it wasn’t obvious to me that it was a leak rather than the suspected flooding. Ah, live and learn.

Thus far, have only taken the truck for a few short drives. It starts and idles well, drives well, but surprisingly (or not), seat-of-the pants power is down compared to the 2-bbl. Maybe not too bit of a surprise since the primary bore size of the 4-jet are smaller that those of the 2-jet, but, I’m still a bit surprised. The motor almost feels/sounds a bit choked like it did with the original 1-bbl.

After some more driving, about 80 miles worth, I filled the tank and checked MPG - 12.75. This is in the same ballpark as the prior Rochester 2-jet and was rather disappointing; I’d been hoping for at least 14. Then, the water pump broke, a mini-saga of its own.

So, I’ll fiddle around some more with the carb situation. I’m thinking the lack of power may be due to the power circuit not operating correctly - the power piston is the only item I didn’t remove/clean out with the rebuild (it’s staked in really well), though it moves and appears to function correctly.

My next decision is which carb to go with. I like the manual choke idea since it’s what the truck originally had and is somewhat unique. I can also fix the leak on the 4GC carb and see it performs any differently than the 4G version that is currently on it. I may take a bit of a break from the carb drama for a bit.

Feedback/comments appreciated, especially regarding the engine’s apparent decrease in power with the 4-jet. Thanks for reading!

['68OrangeSunshine]

Wow! That's interesting. Good luck on your $25 buck Q-Jet.
[EDIT: I answered post #15. Before you put up # 16 and 17. I was not being sarcastic. ]

Leo Santucci's book, ''Chevrolet InLine Six Cylinder Power Manual'' mentions a Brazilian GM stock 292 [4.8L] L6 with a factory intake made for a QuadraJet. Since they use pure cane alcohol for fuel there, the carb is rigged to flow something like 1050 CFM. Sounds like a lot, but alcohol fuel is not as ''hot'' [volatile] as gasoline. So they need a bigger carb.

[AcampoDave]

How do the plugs look?

[67carry-all]

I think pretty good...

BTW, '68 OS: Carb is a "4-jet", predecessor to the Q-Jet. Thanks for the good wishes!

[AcampoDave]

You almost have to try to get the other one going again don't you? A vintage carb like that definitely gives you points in the old school dept. I'm really surprised it didn't wake it up more. But maybe the exhaust manifold is holding it back. It can't suck it in any faster than it'll push it out. If the other one is the same and the timing is good, maybe headers are what you need.

['68OrangeSunshine]

I've had Clifford headers on a 292 in my truck since 1978. Same ones on 2nd engine. They sure help.

Another option is the HD 292 exhaust manifold for the bigger trucks. They have 3 studs to hook up to exhaust pies, not just 2 like light truck 292s share with 250s. They say they flow better. I have an HD manifold, but never ran it. It's on a 1963 292 spare core.

I much prefer Manual Chokes myself.

[67carry-all]

Yeah, I like the vintage carb route as well. I hear what you guys are saying about freeing up the exhaust flow, but I think at this point it's a carb issue as the engine with the prior 2-jet seemed breathe fairly well. I appreciate the info about heavy vs light duty 292 exhaust manifold - I'd thought all the 292 exhausts were bigger than the 250's - more new info for me.

I think the next step will be to swap back the auto choke 4-jet (4GC) just to see if the motor performs any better. Ultimately, I plan to stay with the manual choke (4G) carb, but a temporary swap to the 4GC may help to identify or rule out an issue with the 4G. The engine lights right up with full manual choke, then settles to a pleasant purr within a matter of seconds with reduced to no choke - it's the best this engine has ever started in many, many years.

['68OrangeSunshine]

Points or HEI?

[67carry-all]

HEI. Started with a junkyard pull until I realized the centrifugal advance was partly frozen. Now have a generic auto store replacement for a '78 straight six, coil-on-cap. Initial timing is at 8° BTDC, seems happy there.