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Carbking - I have done a fair amount of reading on building preformance 6 cylinders and have some that make respectable power. My '56 Chevy 2 dr sedan has a 292 with mild head work, quite a bit of cam, headers, 4bbl, 4 spd and 4.56 gears. The carburetor is an Edlebrock 750 which is, from everything I have read, way too much carb for that engine. But the engine seems to like it. It idles well at about 1000rpm, pulls hard to 6800 and the plugs stay a very nice tan color. I started out with a Holley 475cfm and when I added more cam went to a Edlebrock 600. I had the 750 on the shelf so decided to give it a try and, as I said, it seems to like it. The car is very diveable with the muffers connected and with open headers it has run a best of 13.8 in the quarter at just under 100mph. I guess my question is, why would the engine seem to run that well with "too big a carburetor"? Any thoughts?

My guess is that the fuel curve is way too fat for the engine.

While the engine is seeming to run "well", my guess would be that the use of a 500 CFM GENUINE Carter would produce even better 1/4 mile times (probably a reduction of one or two tenths) and significantly better fuel economy. It would not surprise me if a Carter 400 would also outrun the 750 unless you turn a very high redline.

The 4.56 gears are helping, as you probably don't stay below 2500 RPM very long.

The 340 HP/327 used a 575 CFM carb and the 365 HP/327 used a 600 CFM carb (see thread on CFM ratings ). I would find it difficult to believe that you require more air/fuel with a 292 in any state of tune than either of these two engines.

I had a 400 CFM Carter on a 300 CID 6 with stock cam but headers; and it was all the engine could handle. However, I did not have the gears that you do.

Jon.

Jon, thanks for the reply. I guess I assumed that if the fuel curve was extra fat I would see that in the plugs. From your note I assume you feel there is a difference between a Carter and an Edlebrock. The 600 I have is a Carter. I just may through that on and see what it does. We have some 1/8 mile races this weekend. Thanks again.
Ron

Ron - you may need to do some calibration changes on the 600. If you have access to a rod kit, try a rod that has the same high vacuum step as stock, and a low vacuum step about .012 smaller in diameter than the high vacuum step. I am guessing here, so some experimentation is in order.

Unfortunately, the 600, even though it says Carter, is the new style.

Jon.

Jon - I don't mean to be a pest but if I understand all of the above, you agree that the 600 (since I already have it) is a good step to take. It may not be spot on for calibration but there should be a noticable improvement and from there I could pick up a calibration kit to experiment. It seems to run so well I hate to mess with it but if there are significant improvements to be had, I'm all for it! Thanks for you help.
Ron

Ron - and others.

First, you are not a "pest". The only really dumb question is the one left unasked.

Something which many fail to realize is: THERE ARE NO PERFECT "OUT OF THE BOX" AFTERMARKET CARBURETORS.

Your Carter 600 is actually a Carter 625 that was renumbered to compete with another brand's 600. Yours was originally calibrated for an early 70's 400 CID small block V-8.

AS A GENERAL RULE, the Chevy small block likes a huge differential between the high vacuum and the low vacuum step on the metering rods. The Chevy 6 cylinder likes proportionally more fuel at the lower RPM spectrum, thus generally requiring a smaller differential between the high vacuum and low vacuum step. You may also need to change the step-up piston springs if you have a wild cam. The stock springs are probably OK if you have at least 14 inches of vacuum at idle. Less than that, and you should experiment with the springs.

You may see no improvement at all, or even see a HIGHER E.T. if the carburetor is not properly dialed in.

The Carter AFB (MOO) is one of the most tuneable (and responsive to tuning) carburetors ever built; BUT IT MUST BE TUNED!

The general procedure with the AFB is to install the carb in stock form.

Then verify that the step-up pistons are functioning as they should. If not, the springs should be changed to allow the step-up pistons to function with the vacuum available.

The next step would involve the adjustment for steady-state or high vacuum. This should be done using an O2 sensor (or some other form or exhaust gas analyzer). This adjustment normally involves changing the primary jets.

Once the high vacuum adjustment has been made, one needs to adjust the low vacuum on the primary side. This is done, again with the O2 sensor and the secondary side disabled. This adjustment will use the same primary jet, and vary the high vacuum step of the metering rod (changing rods).

Once the primary has been adjusted, re-enable the secondary side and again adjust with the O2 sensor. This adjustment is made by changing the secondary jets.

Each style of carburetor will benefit from a step-by-step adjustment; the AFB should be adjusting in the steps above.

More than likely, the Holley 475 which you had is a more ideal size; but I can't help you with that, as we do not do Holleys, nor will we ever. However, there are plenty of people that do Holleys.

The biggest difference between dialed-in Holleys and dialed in Carters of the proper size (again MOO) is fuel economy. No one ever accused a Holley of getting good fuel economy.

And check my signature about the "wrong" carburetor.

Jon.

Jon, thank you so much. I can't believe how much time you dedicate to this forum. I think I speak for a ton of people when I say we very much appreciate your sharing your knowledge and experience. As an aside, do you ever go to the Iola car show or the Jefferson,WI show/swap? I may have talked to you there a few years ago when I was living in WI.
Anyway, thanks again.
Ron

Ron, it is my pleasure to attempt to answer questions on this and other forums. The old car hobby has been a source of income to me and my family for about 35 years; and has allowed me to build a business behind my home, and basically work from my home. The forums allow me to give something back to the hobby.

We vended for many years at the Iola swap meet, beginning with about the 3rd year. It was the favorite swap meet for us all, because of the friendliness of the Iola people. Hershey was better financially, but Iola was more enjoyable.

We stopped going to Iola when the legislators of Wisconsin, in what seems to me as moments of zero brain wave activity; redefined the term "commercial vehicle". To have continued to vend (legally) at Iola, we would have had to convert the motorhome to CDL specifications; and both my Father and myself would have had to acquired CDL licenses, logbooks, etc. It was not worth the effort. I understand the law was revoked after a few years, but we had already given our spaces to a Wisconsin company and did not go back.

Jon.