I need second opinions before I start drilling holes

Holley 800 (4150) HP 500 carb, mechanical secondaries, choke removed, idle mixture screws on primaries only.

HP 500
9.4/1 compression
Cam retarded 3 degrees
Gil Mag Choice
MSD

Very little response to idle mixture screws

New engine, idle is VERY GOOD when the engine is at normal operating temp

But when engine is below normal operating temp it's loading up at idle and off idle.

At OE setting the primary transition slots are NOT exposed BUT the secondary slots are exposed about 1/16"
I tried closing idle setting on secondaries (slots not exposed) but to get a idle I had to open primaries to the point that primary transition slots are exposed At that setting the primary idle mixture screws worked.

Should I drill holes in the butterflies? What size?

THANKS!

check the timing. the readjust the carb. i think

What power valve do you have? I have the same carb on a 502 MAG and I had to go to a 4.5 from a 6.5 PV. It would idle ok but load up after a couple of minutes. It would respond to idle mixture screw adjustments but it would still run too rich. Someone suggested drilling holes in the secondary plates and someone else suggested trying a different PV. I went that route before getting the drill out and it worked. Engine runs awsome with this carb and will idle all day long now without loading up or sooting the transom.

I have this carb on a couple of 502 Mags also. I thought that they seemed to run a little rich at idle, so this year I adjusted the secondaries close. It seemed to clear up the transom but, it would choke down every time I put it in gear. I adjusted them open enough to get most of that out so I will see if it is also lean enough to keep the transom clean.

I thought that Holly basically had them open to make sure that the secondary fuel doesn't get stale. (They were referencing a car carb at the time). As we all know boats don't have to worry about that.

I thought that the only time that you drilled holes in the secondary plates, is to get more air for a signal on big duration cams.

BTW, I also have 4.5 PV's.

Bob

The retarded cam is probably not helping. Have you run the engine with the cam straight up?