I am working on my buddies 66 Chevelle. 454, mild cam, headers, automatic, mild stall, mild gears...

The previous owner put on a 750 Double Pumper (4779-6). It runs okay but a little rich.

The carb has 4-corner idle mixture screws, and i have never worked with these before.

Can I just seat the rear idle mixtures screws and just use the primary IM screws?

If not, how do I adjust the rears (anything special I need to do?)?

I noticed the rear/secondary Metering block has no powervalve, but a screw in plug in the secondary metering block. Will this matter with the Idle Mixture screws?


I actually thought someone had reversed the primary and secondary Metering Blocks, because there was no screw visible in the Primary block... It turns out there was no screw (fell out?) There is a screw on the other side of the primary metering Block though (maybe its really a 3-corner idle adjustment-ultra-rare :D

that part number is for a holley 4150, which i doubt would have 4-corner idle screws.

just adjust the front ones.

anyways, whether i'm wrong or right, here's how to adjust the idle mixture:

1> a good starting point is to screw the screws all the way in, then back them out 1.5 turns.
2> fire the motor up and set idle rpm to 800-850 for automatic.
3> use a vacuum gauge and shoot for the highest vacuum while adjusting evenly.

all 4 mixture screws are supposed to be set the same. The vacuum guage is the best way to set mixture also, its a little tougher to do by ear.

Tony, never played with a 4 bbl this way, but having 3 idle cicuits avail to me, I do shut down the 2 outboards. Nicer idle and a bit better transition ( not as 'jumpy ), but mpg suffers bigtime. Think if you closed the secondary blades and shut off the idle mixture, it might open up a slight hole in the transition, which you could certainly cover with pump cam/squirter tweaking. It'll be a pig on gas with 4 corner I should think. Slip a 10.5 PV in front and hope for the best. Might need a quick orange pump cam in the secondary side as well.

** I think. ** smile.gif

would you be interested in a 3310 carb with adjustable secondaries? we could work something out. mike

A trade sounds interesting, but not at this time, we are tyring to get this back up and running this weekend (the carb ran okay, just not great. With the mild cam/convertor, I am worried it might load up and backfire blowing the powervalve).

Any replacement would have to have the center hung bowls (dual feed) and dual metering blocks (need both) to fit the "factory" fuel line...