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 (found out one idle screw was missing!).

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 )

Any help advice appreciated... I'd prefer using a Vacuum Secondary for this car (its only a cruiser), but thats out of the budget for now).

Thanks, Tony

Hi Tony,

Start by making sure the floats, accl pump etc are adjusted properly replace the PV so you know you are starting out good. Adjust the throttle blades so you have about .030" of the transfer slot uncovered on both primary and sec throttle bores. The slot is about .020" wide so just a bit more than a square showing from the bottom will do. Lightly seat all 4 rich lean idle mixture screws and back them out about a turn and a 1/4 to start.

This should get you going, be sure to warm the engine up before making any adjustments. Keep the idle below 900 and adjust the rich lean mixture screws for most vacuum. Do all 4 screws and then shut it down and check each. Find the avg of all 4 and set them to it. Fire it back up set the idle. Try to keep both throttle blades the same too. If you back off one a half turn and get the idle you want put it back and turn both a 1/4 turn to them the same...

I have heard of folks killing the rear idle circuit and just running the front but I didn't have any luck doing it. Just remember to keep each of the carbs circuits in mind when trouble shooting and tuning and fix each as a seperate entity.

Check out www.camaros.net/techref/series_2.html (http://www.camaros.net/techref/series_2.html) for some basics and links to some info on the holley web site...

Thanks Dennis...

Doesn't sound too complicated...

Why do the secondaries need the idle circuit?

It's my understanding the 4 corner idle is found on performance carbs with mechanical sec's and sec accl pumps. The idea is to allow more tune-a-bility to help tame that big cam'd beast so it will idle on the idle circuit. Think about some of the "fixes" you hear passed on to get that high performance mill to idle, "crack the sec throttle bores open" and "drill small holes in the throttle bores". Both are attempts to keep the carb on the idle circuit and are kind of doing what the 4 corner does. By using a carb design that idles on both the pri's and sec's you have a better chance of keeping that high performance mill idling.

Thanks Dennis...

But this is just a mild motor... too expensive to replace the carb now, and this one looks almost brand new...

I am going to start rebuilding it tonight (have it mostly torn down)...

Gee Tony, I thought I psoted a reply last night, but it aint here... the short version...

The difference between " 2 corner" and "6 corner" for me is around 3-4 mpg. Sure it idles nicer and the secs don't "jump in" like it does on 2 corner, but that's significant mpg.

Donnow if its the same setup as a 4 bbl but close the butterflies right down, shut down the idle screws. Might have to change to a faster pump cam back there or larger squirter to cover up any hole in acceleration that might develope. If its a mild motor a 105 PV in the primary side will genreally kick on right during the transition to secondary , so that might be worthwhile. It'll suck gas like no tomoorw in 4 corner mode. He'll hate it if he drives anywhere at all.

Here's the thread you posted in Gene. http://www.chevelles.com/forum/ultimatebb.php?ubb=get_topic;f=4;t=021251

Well the right carb for the right application, A mild cam'd build isn't needing a 4 corner carb to begin with. Anything running mechanical sec's isn't going to get the best possible mileage either. I don't know enough about the sec metering block in a 4 corner idle carb but imagine it is closer in design to the one on the pri's then one without an idle circuit built into it. Closing off the sec idle circuit may work on a mild build, I don't know. I was unable to do it on a built 468 that I got to play with.

Gene your 3 two's is a bit different beast, you have a lot more to over come at transition with the 3 seperate carbs. I was wondering why you recomended the 10.5 PV in the other post. If the mill's making good vacuum it won't be opening at idle but it may open too soon when you step on it. It might mask an accl pump problem at transition that should really be cleaned up with a accl cam or squirter adjustment.

I know there are many ways to do things and am just looking for more insight here, not challanging your advice.

Actually, all Holley 4 barrel carbs have a secondary idle curcuit. The 4 corner just allows you to adjust it. On a 2 curcuit system, the rears are just there for the ride and to keep the fuel in the rear bowl from going stale. Not really an issue if you open the sec up occassionally.

On a 2 curcuit system, the rears are just there for the ride and to keep the fuel in the rear bowl from going stale. Not really an issue if you open the sec up occassionally. yep, my holley book also states its there to improve fuel distribution at low engine speeds.

i wonder if its possible to encounter fuel distribution problems when closing the secondary side of a 4 corner idle circuit?

Keep the info coming...

As I said, I KNOW this is the wrong carb for this situation, but I want to maximize driveability until a replacement comes about...

Anybody else have opinions about "seating" those secondary idle screws or opening them... Whats the best for mileage?


I am putting a 6.5 PV in for now, as thats what was in it before (I never measured vacuum, because I wasn't planning on tearing into the carb.). I pulled the carb off when detailing the engine, and thought I'd open it to see what PV was in it, and figured on getting some spare PVs & extra gaskets... Now I've got a rebuild kit...

When you pull the baseplate off of a holley carb you'll notice a transfer slot from the primary side of the idle circuit to the seconday, this supplies fuel for the secondary side at idle on a regular two screw carb. On the four corner models the fuel is also supplied to the secondaries from theat side of the carb and the amount is adjustable with the extra screws. There should be no difference in fuel mileage between the carbs if the idle circuit is adjusted properly IMO. The four corner models are more tempermental with adjustment but definately more tunable. I would start with all four screws out one turn and adjust from there in very small amounts with a tach and vacume guage. Also make sure the air bleed holes in the body of the carb are open or the mixture will be way rich. JMO Bob