68 383 with 750 carb. Runs rich

DJC said
Mine Holley was rich as well, changing the power valve mad a great difference. From Holley Power Valve Tuning - Holley Blog:

"High Performance Engine Power Valve Selection:
High performance engines with modified cylinder heads, long duration camshafts and single plane intake manifolds may require a change to the power valve. To find out which power valve your high-performance engine needs, you can perform the following procedure:

1) Hook a vacuum gauge to an intake manifold vacuum port.

2) Warm up the engine and note the vacuum reading at idle. Automatic transmission vehicles need to be in the Drive position, while manual transmission vehicles can be in Neutral.

3) Divide the vacuum reading in half. The number will determine the correct power valve.

Each power valve is stamped with a number that indicates the correct vacuum opening point. For example a power valve with the number #65 stamped on it, will open at 6.5 inches of engine vacuum. As an example, a vacuum reading at idle of 13-inches, is divided by two and results in a 6.5 inches of vacuum. Therefore, you should have a #65 Holley Power Valve installed in the carburetor.

If you divide the vacuum reading and it falls on an even number, you should select he next lowest power valve number. For example a vacuum reading of 8-inches, divided by 2 and you come up with a number of 4. In this case you would use a #35 power valve.

Finally, if your engine produces 13 inches of vacuum or more, the stock power valve that the carburetor is equipped with from the factory, is sufficient.

Holley power valves come in a range of orifice sizes. The higher the number the more fuel is added."

Ugh. The last sentence of that blog advice indicates Holley is really losing it in terms of technically knowledgeable people. Its too bad.

As for the rest, Holley has suggested using 1/2 of idle vacuum in their aftermarket publications years, although occassionally instead they suggest a PV rating half of highway driving vacuum.
Its one of the few bits of guidance from them that has no connection to engineering or science. Given no other option, maybe its better than nothing.

Why do say this? Because the power valve's job is to richen the main circuits when the load on the engine gets high. In other words as the engine approaches wide open throttle either to accelerate or go up steep hill the mixture needs to go richer. There is no direct correlation between this situation and an engine's vacuum at idle.
Working from high cruising vacuum at least is reasonable because at cruise it definately doesn't need enrichment. We also know that at light part throttle it shouldn't need enrichment - in fact it will want leaner mix. So taking half of cruising vacuum would at least be a logical possibility. The truth is some engines will make good power on a lean mixture up to 80 or 90% throttle (ie low vacuum) and others will need enrichment at only 60 % throttle (relatively high vacuum). And the only way to find that out is testing.

There is one other consideration in PV opening. It must stay open through the top of a WOT run. If an engine is somewhat restricted, or for any reason is less efficient in the run the manifold vacuum will go up. In this situation a really low PV opening (as suggested in the example) will be a disaster.

Getting back to the OP's 3310.
If it hasn't been molested, the PV should have no effect on idle. However there are situations where it does. If you're sure that's whats happening, it can be dealt with if its a problem.