The Stovebolt Site Forums The Shop Area Engine Shop 4 bbl carb for 292

I've searched this topic and found some good info but not exactly what I'm looking for. I have an Offenhauser intake that was given to me and I'd like to find a small 4 bbl carb to put on my 292. I've read the recommendations in many posts but I don't want to spend the money for a new carb and I don't know if any of the recommended carbs are used on production vehicles. My questions is: Is there a carb that would be good for a driver 292 that I can find in a salvage yard? If so, what types of engines should I look for? I don't plan to make any other major modifications, maybe some day I'll get cast iron headers but no lumpy cam or propane pistons or anything like that. Any advice will be appreciated. Thanks.

No, you're about 40 years too late for that. A Quadrajet calibrated for a Pontiac 250 cubic inch overhead cam six might work, but they haven't been made in several decades. Finding any sort of carbureted engine in a junkyard anymore is pretty much of a lost cause.
Jerry

A Quadrajet calibrated for a Pontiac 250 cubic inch overhead cam six

X2, you can start with any QJ in decent shape, but the footprint is (AFAIK) larger than the gasket surface of your Offy - you may need an adapter, or some TIG.
The older (less smog garbage) the better, there are millions of them. If the throttle shaft is tight it can probably be revived with a Tomco kit.
The tune-up will be pretty close to what's in there if it's from a small V8.
You're looking for a primary metering rod around .041" to .044" (leanest) economy step, and a primary main jet between .069" and .072" (richest). The economy step should be about 29-31 numbers higher than the MJ size, e.g. an .041” rod should use a 70, 71 or 72 MJ.
The secondary MJ is fixed. Select a secondary metering rod with a tip diameter of .050” to .060” for a test, such as CK (.0527”). The most common Sprint rod was AY (.057”). The Doug Roe book (and many on-line sites) identify the rods by letter vs. diameter.
The secondary air valve is a simple screw adjustment to a spring, if it bogs tighten the spring until it stops.
There's much more to it, but that should get it running.
Because of 292 size, and safety, you should start on the rich side.

A QJ for a 250 will run just a bit rich on a 292, due to the extra air volume of another 40+ cubic inches flowing through it. On the other extreme, a carb for a 305 Chevy or a 307 Olds V8 will run very slightly lean since a bit less air will be flowing. At low to moderate RPM, the jetting should be pretty close with either option. The Quadrajet will be a much better choice than a small-bore Edelbrock or something similar, even with an adapter plate, because of the small-diameter, 3-step primary venturis and the massive secondaries with the air valve control to avoid over-carbureting the engine at WOT.
Jerry

I have used the Holley 390cfm 4bbl carb on several sixes and small V8's through the years with acceptable results. I am not a big fan of Holleys for street use but the small 390 hasn't given problems. When I checked Jegs's for price my jaw hit the desk---$465. The old Ford/Autolite 4bbls for the 289/302 can still be found and are a very simple and dependable carb but not especially high performance.

The Offy 6 intakes that I have seen are set up for a square-bore carb.

The Q-Jet is a spread-bore, thus the need for the adapter as mentioned by others in this thread.

Carter used to make an aftermarket AFB that was specifically calibrated for a 283 Chevy in a 400 CFM size. Two were made: part number 9400s (with no EGR provision), and 9410s (with EGR provision). I had a 9400s on a 300 CID 6 for over 400k miles with excellent results on our shop truck, so personally know these things will work. Believe it or not, the 4-barrel Carter got almost DOUBLE the fuel economy of the 1-barrel Holley it replaced.

The big issue is price. No, they were never used as original equipment, and you probably have a better chance of finding a needle in a Kansas hayfield than finding one in a junkyard.

They do occasionally surface on Ebay, but never cheap (if a decent core).

But the Q-Jet for the Pontiac 250 is a collectors item, and not inexpensive either.

Generally speaking, the cost of rebuilding the Carter is much less than rebuilding the Q-Jet. 90 plus percent of the pre-1975 Q-Jets are going to require both primary throttle bushing, and a heli-coil in the fuel inlet. While hobbyists try these operations, generally the hobbyist fails and ruins the carb. Both procedures should be done by a competant machine shop (or a hobbyist who has access to, and can operate a vertical milling machine). I have yet to see either procedure required on any AFB.

Another possibility would be a Rochester 4-Jet (the 4-barrel from Rochester prior to the Q-Jet) for a 283 Chevy. They are square bore, and approximately 400 CFM. Also, they have a cast iron throttle body (unlike the aluminum Q-Jet T-body) and virtually never need bushing. The fuel inlet also rarely needs attention.

Since most enthusiasts would rather have the Carter WCFB over the Rochester 4-Jet on the older 283's, you still have a ghost of a chance of finding a rebuildable core in an older yard.

Jon.

I'm using a Weber 36/32. It's a progressive two barrel electric or manual choke.

It's probably my upbringing. I started life driving a MG midget. And suddenly found the Triumph TR4A. Which I owned several

These carbs cost about $ 600 But a Salvage Yard????

I am not a fan of Holley for street use but the small 390 4bbl carb has been used for years on six cylinder and flathead Fords. It can still be bought brand new and parts are available. It is a square bore so no adapter needed.

Been looking into running a 4 barrel for my 235 as I have a nice Clifford intake, looks like the Holley 390 cfm is what I'll be running. That said, a friend went with this carb on a 300 ci inline Ford and found it was to small, moving on to 500 cfm I believe.
Ran the 390 cfm on my old 194 Nova and WOW, what a performer

Best of luck.
Also may want to check out this site for 292 performance info, interesting stuff.
http://www.12bolt.com/

Would a Carter AFB off of a 4.3L GM marine engine work ok?

???

Race or street?

CFM at 100 % volumetric efficiency = (CID x RPM) / 3456

Rewritten RPM = (3456 x CFM) / CID

Or, for the 300 CID Ford

400 CFM is sufficient for 4608 RPM at 100 % VE

Or at 90 % VE 5120 RPM

I had a 400 CFM on our 300 CID, and it was a bit large, but still worked OK. The secondaries never opened in 4th gear.

Jon.

Would a Carter AFB off of a 4.3L GM marine engine work ok? I found an instruction sheet on the net that stated not to use automotive carbs on marine engines but did not mention visa versa.

Which AFB???

And which GM marine engine???

I have no record of an AFB used as original equipment on a GM 4.3 marine engine.

Jon.

It's on ebay. http://www.ebay.com/itm/Carter-afb-carburetor-/321914781780?hash=item4af39dc054:g:nvsAAOSwYHxWPn9m&vxp=mtr

Just says Carter AFB competition
"came off a gm 4.3L mericruiser"

I've asked if there are any other pn's but no response yet.

It may have been on a 4.3 Mercruiser, but Mercruiser didn't put it there.

Cannot read the number, so cannot tell exactly what it is; but that style is a minimum of 500 CFM.

Something else to consider: I know it says Carter, but that is an FM clone, not a real Carter AFB.

Jon.

In my experience, an explanation for success or failure of carbs in non-calculated sizes (bigger or smaller) is that the new fuel curve was more (successful) or less (failure) suited to the tune-up.

Considering the possibility of a Weber 38/38 2 bbl now. I see prices on the internet that are all over the place. Even if I filter out the ones that appear to be copies and only look at the ones advertised as genuine Weber carbs there are still big differences in price, with the speed parts vendors being much higher than others. Are there differences in the parts that make one more valuable than another or are they all the same? What about the generic ones and the used ones removed from Jeeps etc?
Thanks again.

Will leave questions concerning Webers to others; I have had less than wonderful, but expensive, results when trying to use them on other than original factory installations. The factory engineered systems, at least on cars in the 1950's and 1960's that I am familiar with, worked well.

Jon.

I was able to scrounge up a Holley 2 bbl 350 cfm for $100. Don't know if that's a good deal but I think it should serve my purposes and it's a lot cheaper than a 390 4 bbl. I would like to have found a Holley progressive 2 bbl but they're pretty scarce.

The 350 Holley 2-barrel should be an interesting stepping stone between the original Rochester 1-barrel and either the 390 Holley 4-barrel or the 400 Carter 4-barrel.

Remember 1 and 2 barrel carbs are rated differently; so comparing the 350 to 390 or 400 is comparing apples and oranges.

The original 292 single barrel Rochester was rated 250 CFM on the 1 and 2 barrel scale.

Converting the 250 (original) and the 350 (Holley 2 barrel) to the 4-barrel scale results in:

Original Rochester 1 barrel - 176
Holley 350 2 barrel - 247
Holley 390 4 barrel - 390
Carter 400 4 barrel - 400

Is your Offy an open plenum, or does it have 4 holes for a 4-barrel?

Jon.

It's got 4 holes for a 4 bbl. I might just use the stock manifold with a 2-1 adapter and save the offy for a later project. The Holley 350 is recommended as a good upgrade in the Santucci book. I might be satisfied with it after I finally get it going. Thanks for all the help.

The racing association I participated in for several years required all the cars to run the Holley #7448 350 carburetor as a method of preventing radical modifications. There's no point in doing high dollar modifications and then choking the engine down with a small 2 barrel carb.

A 350 Chevy will run a maximum of about 6100 RPM at wide open throttle with that carb, regardless of the gear ratio or cylinder head modifications, camshaft, or other engine building tricks. That RPM converts to 607 CFM, which is obviously way above the theoretical capacity of the carburetor. I wish I had recorded the manifold vacuum on one of the dyno runs we made with that carb- - - -I'm sure it would have been higher than the vacuum reading where the theoretical CFM was calculated!
Jerry

If your vehicle is currently running with the original Rochester, you might try:

(1) Run three 0-60 timed runs with the original carb, and compute an average.

(2) Add the two-barrel with the 2 -> 1 restricter (er, adapter).

(3) Run three 0-60 timed runs with the 2 barrel, and compute an average.

(4) Compare the average computed in (1) with the average computed in (3).

(5) Then you know the effectiveness of your change.

Jon.

Yesterday I was checking out Craigslist and ran across a guy who is abandoning a project with an old Ford that's too rusty to save. He was selling off some parts he has collected for the project and had a Holley 8007 390 cfm, new in the box. He sold it to me for $150. I had to make a little road trip to get it but I think it was worth it. This should be just what I need. Thanks for all the advice.