Secondary Power valve.
 

I recently installed a Quick Fuel vacuum secondary adjuster to replace the Holley spring one that came with the carb. It works great. Prior to doing so, I removed the secondary power valve, installed a block off plate and bumped up the secondary jets to 8 sizes. A lot of documentation out there about the pro's and cons of doing this. I like the way the car performs without it.

Fred

Sounds silly but it works
 

Ive got a carb cover thats intended to cover the intake while fooling around with things with a large air cleaner removed...the stouter the better.

I read about this on one of the hot rod forums some where; run the engine at idle and up to temp, then put the hat over the carb. The vacuum suddenly pulls hard on all those little passages, the idle feeds, transition slots, etc...i thought about it, doesn’t seem like it would hurt anything, did it and the carb seems much happier...downside is whatever little POS was in there just got eaten by the motor...ah hell thats why motors come out.

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?

If you do have a 4150 double pumper, close the primary side to make the transfer slot a "square", and then open the secondary side to about equal by viewing with light, (carb removed from engine).
Refit carb, DO NOT touch the primary side for speed adjustment.
Use the secondary speed screw for idle speed.
Do not drill holes, no need for that old trick.
Gary

Moriarty, after you follow what Gary has suggested, you may also want to install a lower power valve. If you’re pulling only ~6 inches of vacuum at idle and currently have a 6.5 valve on the primary side, it’s already bleeding and you probably have a very rich idle condition as a result. But adjust the throttle blades first to determine what your corrected idle vacuum is before throwing parts at the carb.

Ah, good call. Now, I just need a trick for adjusting the secondary screw while on the vehicle - it's only accessible from the bottom of the base plate. I think I'll start by just eyeballing the difference with how much I lower the primary to get it back to square.

Re: vacuum - yeah, I'm hoping to get it up to around 8-10, so I'll focus on that before touching the power valve.

Thanks!

UPDATE: The mechanical linkage between the primary and secondaries prevents adjusting the secondaries to expose the transfer slot at idle, so that kills that option. I played around a bit this afternoon and have things working a little more to my liking now: bouncing between 7-8". This cam wants lots of idle advance - I get ~10"Hg @ 25 degrees. I've been looking into programmable igition solutions, so that might be doable, but wanted to make sure I got the carb in good shape first before messing with that.

Put that Thing on gumtree purchase a Holley sniper kit and never look back . I will never waste another dime of my money playing with carburetors. The gas savings alone are worth it ,and performance wise not even a comparison.

Holley sniper with hyper spark ignition. Best $ ever spent on my cobra