68 383 with 750 carb. Runs rich

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."