Holley carb on my Gremlin

tsanchez wrote:

The size of carb has little to do with how it idles,


Actually this statement is not even close to correct.
The size of carb when it is the wrong size has everything to do with low rpm performance.

It is the air flow through the carburetor that meters the fuel it dispenses and all kinds of driveability problems can be incurred when the wrong size is used.
When the carburetor is too big, the air flow through based on how it is metered is too little.

I ran a 750CFM AFB on small six and yes it idles "ok" and I could get around things, and it ran oh so much better when I got the right size AFB.

That one worked out of the box and I did not have to screw with it.

So if you have the choice, get one that is the right size. Or be willing to put up with some crutches to have it run "ok".

Or

be able to implement the crutches.

GM put a 760 CFM Quadrajet on the 230 Cu In Overhead Cam engine in the Pontiacs, but they had Rochester Products behind getting it tweaked to work. And a Quadrajet has a LOT tweakability.

Most of the Carburetors in use these days have Factory Owners Manuals floating around on line that can be down loaded and used for informational purposes. Like for adjusting and tuning.
I have one for every carburetor I own, along with a 1976 manual that lets me chase wiring Problems out. AMC used a lot of GM wiring and GM covered it better. I also have a 1976 Chrysler Manual which does a good job of covering the ThermOquad, which after all was a MOPAR design and I am using that on the V8 engine in my Hornet. Boy howdie that is a well designed but complex carburetor. And I picked up a spare one for parts that last was on an international V8.
And when you go through those manuals they will cover the problems that can be had when a carburetor is miss-applied. And why!

The 500 cfm 2-bbl shouldn't be as big as we think, due to the difference in dP that Holley uses to rate 2-bbl carbs versus 4-bbl carbs.

But, it's got the same throttle bore size as a 750 cfm vacuum secondary (1.688" bore). That's a good indicator of the size of dry air slug that you need to compensate for with the accelerator pump tuning.

I've tuned the 600 cfm vacuum secondary (1.563" throttle bore) to run well on 258's lots of times, but the jump to larger bores might make it really tough. The pop and stumble on initial acceleration are going to be related to the accelerator pump shot volume and timing.

The cold start and warmup issues are fixed via choke adjustment, assuming you also make sure the idle mixture is adjusted correctly.

I would also look into your ignition timing. You probably need to run more initial advance than you have now, and a more aggressive advance curve. Both will help here and make more power for you.

If you can get the 350 I'd do it though as it seems a much better fit. It uses a 1.500" throttle bore.

I think it depends on where your throttle blades are now at idle. If they are into the transfer slots, running manifold vacuum will increase your timing at idle, thus raising RPM's, allowing you to close your throttle blades to where they should be. How many turns out is your idle screw?

That was me that recommended you switch the distributor vacuum to non-ported. 

Always use manifold vacuum. The lowest on the carb.