GrandSportC3

How do I adjust the idle mixture leaner... Do I have to turn the idle mixture screws left or right to get a leaner idle mixture?

:cheers:

Olivier

rainman69

Depends on the carb. Whichever way backs the screws out of the seats is richer. The way to set it is to first screw them in until they LIGHTLY touch the seats. Then back them out 1 to 1 1/2 turns. Check your idle mixture with the engine hot.

norvalwilhelm

Holley idle screws are meant to be set at 1 1/2 turns. Sure there are fine adjustments but they should end up about there. With a hotter cam you might have to drill 1/8th holes in the butterflies that have accelerator pumps. A double pumper gets 4 holes. They must also be drilled on the side that forces you to hold the throttles closed while drilling. If you can just turn the carb upside down and drill they are on the wrong side of the shaft.
You can also do the .015 wire trick to the idle hole to lean out the mixture.
If you screw the screws in or clockwise nearly all the way to lean it out you should be doing something else, like the holes and wire in the circuit.

Steve Straus

I usually get the same effect by adjusting the secondary stop screw so it cracks the rear butterfly open just a little.

GrandSportC3

Well - my carb is the 80528 HP Series 750 double pumper (mech. secondaries) with Dominator Fuel Bowls....
Currently - the idle is set to 1200 RPM when cold... When the car gets above 195 degrees - the idle drops to 900... sometimes the engine even floods... :( :( :(

:cheers:

Olivier

GrandSportC3

I actually have 4 corner idle on my carb... So - if I understand correctly - I have to screw all 4 idle adjustment screws completely inside and then 1 1/2 turns out.. right???

Thanks!!

:cheers:

Olivier

L79vette

Right. Clockwise gently until they seat, then back out 1.5 turns. This is a good starting point. I like to run the idle a bit lean to reduce fouling.

Mine right now are only 3/4 of a turn out, but this is a 4150HP carb which may be a bit different. I do have the holes in the butterflies.

norvalwilhelm

Steve Straus you can open the secondaries a little but in this case it would cause the transfer slots to be uncovered which screws up the idle circuit. You need to close the throttle enough to prevent fuel from being drawn from the transfer slots. In this case the , start at .100 in you like holes in the throttle plates which allow the throttles to be closed protecting the transfer slots. If this still isn't enough the wire in the idle circuit is needed. Holley even sells throttle plates with the holes already drill but you can do your own. Holley also recommends the wire in the idle circuit.
If you need to close the idle needles off almost completely to get the car to idle you need to do something and that something is close the transfer slots. They are being exposed.

Barry's70LT1

You may also need to increase the size of the Idle air bleeds. DP's usually have very small holes also causing a rich mixture.

GrandSportC3

Great information guys!!!

Thanks

Olivier