I'll re-post my post from the Hardcore section since it may be relavant here. Maybe the carb is too big. I once had a smaller quadrajet on my 283. It needed rebuilding buy I had a fresh AFB off of my 65 Lincoln w/ a 430 cu in engine. With the adapter plate and the larger carb, I was a bit overcarbed and always had a dead spot.
So I throw these thoughts out for fodder.
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Since we have been discussing venturi sizes in the little six forum, consider this:
Maybe the Holley 500 just has venturi’s that are too big for these engines. A 200 cu in engine at 5000 rpm with 100% volumetric efficiency requires 289 cfm of air.
When Mike R did the carb swap out at Pony Carbs, the Holley 500 cfm made 177.7 HP. The Autolite 4 bbl 480 cfm made 211 HP and had a very flat torque curve. The Holley had venturi diameters of 1.43â€. That translates into a total area of 3.21 sq in. The Autolite had venturi diameters of 1.00 and 1.08. That gives a total venturi area of 3.40 sq in. That is nearly the same area between the two carburetors. But, flow is divided between 4 venturis instead of t2 larger ones. Based on the 289 cfm of air consumption, each bbl in the Holley has to provide good fuel metering over a range of air velocities of 0 to 6,480 ft per min. On average, each bbl in the Autolite 4 bbl is handling a range of air velocities from 0 to 3,060 ft per min. Now this is just an assumption, but I suspect that the 4 smaller venturis do a better job of metering the fuel probably because they are operating over a smaller range of air velocities. I suspect the larger primaries of the Holley are not metering fuel as accurately. The Holley will have a larger range of pressure differentials associated with the large range of air flow. However, fluid flow through an orifice or jet is not a linear relationship to the pressure. It is a logarithmic function. The jets in the Holley are being asked to try and deliver fuel at the same AFR over a wide range of air flow based on a pressure differential that is changing logarithmically.
Doug
