I have a Holley 450 CFM carb that came on my 66 283 chevelle when I bought it and i was wondering if they are any good for small displacement - SBC engines? I'm running a 332 now and depending on what the VE is at higher RPM's, this carb may be a little on the small side but it should make decent torque and snappy throttle response. The only reason that i'm considering it is because I have tried a quadrajet and two different holley 600 VS carbs and all seem to run rich. The spark plugs are always black and sooty around the base thread ring. The porcelin is bone white and the strap is clean to just below the bend. The engine makes around 12" of vacuum at idle and I have a good 6.5" power valve(not blown). Is anybody still running one of the 450CFM carbs or does anyone have an opinion of them? Does anyone know what could cause the rich condition? The 600 CFM carb has 65/69 jets and the stock VS secondary spring.

Clyde

600 should be fine on a 327.

A smaller carb will not necessarily change "running rich", just because it's smaller. You control that with the jets and various other things. It's entirely possible that the smaller carb would run even richer.... depending on how it's set up. In fact I'd say you have 50% odds that it will be richer than what's there, since it probably won't be identical.

I'd try calibrating the existing carb to match what the engine wants, before just yanking it and swapping another one on.

I don't have any bogs that would indicate a starting point for carb tuning so is there any rule of thumb that I could use to start. Based upon what I have read here, it would seem that there is an inverse relationship between jet size and vacuum but is it linear enough to act as a guide? In other words, generally you need a larger jet size to compensate for a lower vacuum, so for a given idle vacuum, can one generalize about what jet size is a good starting point? Without an O2 senser, how do I determine at what RPM's it is running rich or lean?

Any ideas would be appreciated.

Clyde

No, idle vacuum has absolutely nothing whatesoever to do with jet size. They are not related in any way.

If this is a street car, go buy either a Holley "trick kit" or an assortment of jets in sizes larger and smaller than what you have now. Then get yourself a pencil and paper so you can take notes as you go so you can write down everything you see or change as you go - jet sizes, PV, vacuum readings, etc.) - and try this:

Set the floats correctly first. Adjust the pri float until gas just barely doesn't dribble out by itself, but if you bump the fender of the car, it will just barely slosh out. Adjust the sec level so you have to shake the car pretty good to get it to come out.

Lower the jet size in 3-number increments until the car just starts to surge while cruising in high gear at about the lowest reasonable speed for such a thing; like you might normally shift into 4th at 40, so test it at 45, at a steady speed at 45. Then go back up 2 sizes. So for instance if you discover that it just starts to lean-surge at 66 jets, put 68s back in. Then, watch your vacuum gauge, and note the lowest vacuum you see when either idling in gear, or cruising as described; and put in a power valve 2 numbers lower than that vacuum. So for instance if it idles in gear at 12" and cruise at 14", you'd want a 9.5 or a 10.5; if it idles at 15" and cruises at 15", you definitely want a 10.5; etc.

Then set the idle speed to a reasonable RPM, like 800 out of gear or so. Take the carb off, turn it upside down, and look at the transition slot. It's a vertical passage in the front of the throttle bore, right in front of the throttle blade. Adjust the idle stop screw until only a very small amount, like .020" - .030", of that slot, is exposed below the bottom edge of the throttle plate. Then, however much you adjusted the primary throttle to get it like that, adjust the secondary idle stop screw so that you move the sec blades about as much as you moved the pri blades to get the above result. So for example, if you had to back the idle stop screw out, and you saw the pri throttles close about .050", then open the sec throttles by about that amount. Then put the carb back on, set the idle speed again, and asjust the mixture for highest idle vacuum, then reset the speed, then the mixture, etc. Turn the car off, run the mixture screws in GENTLY while counting how many turns out they are, and back them out to the average of the 2; for instance, if one is at 1¼ turns and one is at 1¾ turns, back themn both back out 1½ turns. Start the car, go through the mixture and idle RPM adjustment one more time. Take the carb back off and see where the blades are with respect to the transition slot. Go through that procedure until you get the slot just barely uncovered as described, the highest idle vacuum, the right idle speed, and the idle screws within ¼ turn of each other.

Then change the plugs, go drive the car, and see what the plugs look like.