A good friend of mine Mike, is a very knowledgable performance guru and we got to talking about Holley carbs the other day after doing some tuning on my Son's Holley. Mike was telling me that the idle mixture screws on a Holley are for adjusting the amount of air entering the idle circuit on a Holley, he said only carbs with baseplate located idle mixture screws meter fuel. Mike also said clockwise for richening and counter clockwise to go leaner.
This is totally bass ackwards from the way I know. In my opinion the only way to adjust the air/fuel mixture within the idle circuit is to change or modify the main body air bleeds or the fuel bleeds located in the metering plate. The mixture screws are for adjusting the amount of fuel dispersed through the idle holes below the throttle blades.
The only Holley I've seen with an adjustment for air into the idle well was an old holley 780 that had two adjustment screws located near the choke tower. I have also seen some spreadbore models that had idle mixture screws that worked in reverse, clockwise for lean, counterclockwise for rich.
So, am I right here or is Mike?

You are correct. The idle mixture is determined by the idle air bleeds in the meter block. The metering block screws only determine how much (or volume) of the mixture the engine gets. Many people think the screws alter the ratio of fuel to air. That is not correct. Monte

From what I'm read, the idle screws only alter fuel volume, so to me that means the air to fuel ratio is constant. To change the air to fuel ratio the air bleeds or emulsion jets(I think thats what they're called) must be changed. I don't understand why though.

There are some rare Holleys with different idle circuits. Screw out = less gas. But I think these were emmissions carbs.

Steve

Now can someone clear something up for me?

When people say 1 1/2 turns out for a base adjustment does that mean 540 degrees CCW or 270 degrees CCW?

The Holley 4010 and 4012 idle screws are opposite. These were 'performance' replacement emmissions carbs. The are the non-modular style with no metering blocks or plates.

On these carbs turning the screw in adds volume and backing it out reduces volume.

Originally posted by Slowpoke70:
Now can someone clear something up for me?

When people say 1 1/2 turns out for a base adjustment does that mean 540 degrees CCW or 270 degrees CCW? 540 total .. 1 1/2 complete turns of the screw

Originally posted by Slowpoke70:
Now can someone clear something up for me?

When people say 1 1/2 turns out for a base adjustment does that mean 540 degrees CCW or 270 degrees CCW? 360 is one turn plus 180 = 540

There were on some " emission " style 1850 based carbs that were reverse on the idle mixture screws also.

http://www.holley.com/HiOctn/TechServ/TechInfo/TI-219.html

Thanks

I'm Mike ... ha ha ... thanks also graemlins/clonk.gif

Hi Mike, you had me confused for awhile :D