Whoever suggested to take a look at make sure the power valve isn't hitting the boss in the main body is a good idea. Many of them hit that boss, and it will allow a big fuel leak that will drive you nuts.
If that is not the case, the next thing to look at is the Idle Feed Restriction in the metering block.
I looked at a 750 Holley a few weeks ago that had HUGE (and by HUGE I mean MASSIVE) IFR's in it. They measured with pin gauges at .060 and they should be .035-.037ish. So almost double the correct size.
If you have 4 corner idle, you need to verify both metering blocks for IFR.
If you find they are sized wrong, you can use brass set screws to correct that.
Also verify the Idle Air Bleed sizing.
