Vacuum Secondary vs Mechanical Secondary

[DanEC]

You should be able to tune the idle circuit with the mixture screws provided your cam isn't too big and you have a reasonable idle vacuum reading. Are you tuning the idle mixture on both carbs together to maximize idle speed - it's a rather tedious back and forth process?

I start by closing the front throttle blades until the cam is just barely making contact with the idle screw. Then open both carb idle settings about 1-1/2 turns equally and start the car to make sure it has a reasonable idle speed. If it needs to increase or decrease, then make adjustments equally on the idle speed screws for both carbs. Then I use a tach and a vacuum gage to start tuning the mixture screws on one carb at a time to maximize the readings - go to the other carb and repeat - go back to the first carb and readjust - then to the second carb again. There is going to be a sort of dead zone in the mixture settings that don't seem to make any difference in the idle speed or vacuum readings and I tend to set final adjustments towards the lower side of that zone to attempt to be slightly lean - but I'm not sure it really works that way or makes any difference. The system just may not allow such fine adjustments. After you have the idle mixtures optimized, turn both idle speed screws equally to adjust for the curb idle speed that you want. As long as your idle mixture circuits respond - roll off in rpm on the rich side, and roll off in rpm on the lean side - then all should be good with that.

However, the moment you crack the throttle the transition circuit comes into play (not the accelerator pump circuit) and it is pre-set in Holleys by non-adjustable air bleeds. And if you have a big cam and a high idle speed for it, the transition circuits may even be contributing at idle and if so, other than lowering idle speed, there isn't much you can do about that without extensive carb mods. These transition circuit bleesds are set as a compromise by Holley to work with just about anything that you might bolt that carb on from a 350 up to a 482 stroked FE. That is what is probably rich on your car. There are means of modifying the bleeds to lean out the transition circuit - you can do some Googling. They run from inserting pieces of wire in them, to drilling and tapping for replacement bleeds, to relocating the bleeds. Or some of the premium Holley knock-offs like Quick Fuel had replaceable bleeds - but Holley has bought most of them out.

But no matter how much you work on one of these, a carb set up is never (well, that may be too strong) going to idle as cleanly as a modern fuel injection system.