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Old 06-27-2021, 12:15 PM
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Tom Vaught Tom Vaught is offline
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Most Holley 750 cfm 4779-1, 4779-2 thru 4779-5 were TWO Corner Idle carbs.

Same deal for the Holley 4781 850 cfm early carbs.

The carbs did have a pair of tiny fuel discharge holes under the transfer slots on the secondary side of the carb to let a bit of fuel get purged from the secondary fuel bowls if the owner was not opening the rear throttle blades constantly.

So then about Holley 750 4779-6 Holley added the 4 corner idle circuits to allow using a common metering block on the front and the rear of the carb to make more fuel flow thru the rear idle circuits of the carbs.

The OLD setting on a carb for the primary 2 corner idle mixture screws was 1 and 1/2 turns out from seated. The 4 corner carbs spec was 3/4 turn to 1 turn out for those carbs. Supposedly this helped the fuel distribution in open plenum Holley Intakes. The total idle fuel supplied to the engine was the same in both carbs.

The only Throttle Blade adjustment is to set the Front Primary Blades so that you have a square window when you look at the carb from the bottom and the Primary Transfer Slot. Then you never touch that idle speed screw on the base plate again.

The secondary blades need a slight modification. You make sure the blades are centered in the bores when closed and then you flip the secondary throttle blade "stop screw" so
that you can adjust it from the top. Screw Driver "slot" now goes on top (pointing at the sky) and you can adjust the secondary blade position to get the best idle rpm for your engine. Most times you can leave that screw alone too. So from now on THAT SCREW is what sets your Idle RPM.

Hope this helps.

Tom V.

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