Which Holley do you have? Reply with the list number stamped on the drivers side front of the choke air horn.

It's usually a 4 digit number with a dash and another digit after that. Example, 4777-2
Then under that is another set of numbers, those are the manufacture date.

The 509 with its 240+ degrees of duration is much too large. On a mild 318 I'd be looking for a camshaft with a maximum of 214 degrees intake duration at 50 lift, and would try get right close to 210.

Engine speeding up as you turn in the mixture screws is the classic sign of power valve problems. I think moper has the right idea. Pull the front fuel bowl and the metering block. Replace power valve with a 4.5. It'll help either of two ways:
1. Replacing a leaky PV diaphragm with a new one, or
2. Keeping the power valve closed at idle.

R.