slantscamp,
This carburetor is just too big for a stock (no head work, or warmed up cam) slant. You are not getting enough vacuum signal off idle, in other words not enough air flow at low rpm to atomize the fuel sufficiently creating a lean condition & bog.
I have a 390 Holley and understand your tuning frustrations, but that carburetor is a bit too big for my application with a ported and shaved head, race cam as well. But I march on with it because it looks cool, sounds cool, and it is mounted in a summer toy that dose nothing more than absorb car wax. If I had any sense, I would dump both 390 & high rpm cam for a torque grind, and a nice two barrel, loose the duals and head banger turbo mufflers and most likely pick up five more mpg, more usable power at highway speeds… But why??? Might as well paint that car beige and slap a Toyota badge on its nose while I’m at it…
From what I have gathered from reading about this stuff from Josh, both on this site, and other places, the higher the air velocity through the venturi, the easer it is to tune a carburetor. There are several was to accomplish this if you can think of your engine as an air pump. Larger valves, higher compression, more displacement, and a different cam, and a higher red line will enable an engine to pump more air per unit of time... CFM
What you did was make a bigger hole with that 500, than your 390 had, so air velocity dropped across the full rpm range in comparison.
This is why the super six two barrel configuration works so well. It is a smaller carburetor that better matches air flow requirements of a stock 225 cu inch engine at red line.
Now if you had a slant that could crank to 8000 rpm & idled at 2000 rpm, that 500 cfm unit would be just the ticket. Maybe a 600…
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