First, I know this may have been covered here before, but I have gathered very specific diagnostics that I believe if someone out there has run across this before could lend some advise.

Engine - 383, 1970 LT-1 block, 10.5:1 flat top pistons, reworked 461 iron heads, 290 duration, 490 lift, solid lifters, msd ignition, rpm air gap intake, holley 750 street hp dp

Timing - msd vacuum advance dizzy, timed all in at 36* by 2500 rpm's, 18* initial

Vacuum - 13" hg at idle, has been as long as I have run the engine

Installed a 6.5 pv, installed the DP and set the float levels, adjusted the accelerator pump arms, tried to set 4 corner idle mixture screws. This is where it gets interesting! The primary idle mixture screws can't go past 1/2 to 3/4 turn without the engine idling rough, and very rich. The secondary idle mixture screws can be adjusted like normal, screw in the engine bogs, screw out the engine gets rich.

I decide to leave well enough alone and take the car out for jetting, doesn't require the idle circuit. I run the car hard through 3 gears, shut it off, check some plugs. Started with a 75 main jet, ended up with a 70 and nice clean plugs at the end of a run.

I called Holley Tech Support about the idle mixture screws, and the nice man on the other end of the line says I need to crack the secondary throttle plates to just where I see the idle transfer slots, reset the primary idle speed screw and start over. He's thinking I have the primary idle transfer slots exposed, making the primary idle screws useless.

I do this, and nothing, same deal, I can screw the primary idle mixture screws in fully and the engine does not change speed. If I go past 3/4 turn, the engine begins idling rough and very rich.

So, this is where I am in the tuning process and thinking there may be something I have overlooked?

Any assistance would be much appreciated.

Frankenvette

 

If you can screw the idle screws in al the way and it doesnt change, its getting fuel from some where else. Make sure theres no fuel coming out of the booster. Some times they'll drip from too high a float level.
Can you close the primary and secondary mixture screws and make it stall?
Have you tried starting at 1/2 turn out on all 4 corners, and going in 1/16th turns, to find the sweet spot?

 

edcmat-l1, thanks for the response. Got a couple things straightened out today. Changed fuel pump to 110gph, just to be sure. Reset the primary and secondary throttle plates to below the transfer slots. Got the engine running and tried screwing both sides on the primary and secondary closed, and the engine died. Checked the float levels, all o.k. Started at 1/2 turn in and moving in and out just until the vacuum changed. I ended up with all 4 corners at or around 5/8 turns. Car seems to be running great on the upper end. Got a slight hesitation off idle, but can get that out with some tuning of the accelerator pump, and possibly a larger squirter.

 

Whos carb are you using? you may need to drill the emulsion tubes to make the boosters more sensitive.

 

Holley 750 Street HP, double pumper.

 

You need to drill .110 hole in each primary throttle blade. It is like inducing a vacuum leak to lean out the idle.

Some people also place a fine wire in the idle port to restrict the fuel flow.

As foot note I have a stock AC-delco mech with return line pump and my motor runs fine under very demanding road racing at 5 psi or less in a 820 cfm carb

You also might slow the advance to all in at 3000