[Moriarty]

I have a Holley 4150, but seems like this all applies equally.

Never worked on carbs before, so this is all new to me.

I have a big cam in a 427 SO, and am only pulling ~6"Hg at idle currently.

When I went ahead to adjust the primary throttle plate to only expose 0.030" of the transfer slot was when I discovered that this is what the idle speed screw controls.

At this setting the engine idles at a very low rpm (less than 500). To adjust up to ~900rpm takes five half turns, the first four of which don't have much effect at all. I think I was bottoming out on the automatic choke, and wasn't quite up to temp yet. I'll have to do some more testing tomorrow at operating temperature.

Anyway, this (obviously now) exposes more of the transfer slot than recommended. I'll need to take the carb off to actually measure.

I've seen lots of recommendations online that for big cams / idle under 1000 you often need to drill holes in the primary plates. I'm fine with doing this, I just want to be absolutely sure that's the correct approach since the operation is not reversible.

When I adjusted the primary plates, I noted that the secondary plates were fully closed. It sounds like I should open them up a crack, just to the point where they are completely covering the transfer slot. Not sure if that'll make any difference, or just improves responsiveness when they open.

The carb currently has a 6.5 power valve installed with 65 jets in the primary and blocked off secondary with 80 jets. Should I swap out the power valve for a "big cam" 3.5 first and see how that changes things?