750hp series holley. when i first sarted the engine a few weeks ago, plugs would go black almost instantly. replaced the 6.5 power valves with 3.5. this helped, but has not fixed the problem. 15-20 minutes of running and there black. The jetting is 73 all for corners. Do i need to drop the jetting down and if so how much? When i start the engine, with the bay doors up and the rear of the car out, in a matter of 2 minutes i cant breath or see. its so bad that an hr. later i still feel sick. ideas, thanks guys Dave

Dave, where are your idle mix screws set?

John. i set the screws to the highest rpm. they are about 3/4 turns out. there does not appear to be any rpm increase after that. i tried to use a vacuum gauge but vacuum is so low that its hard to see the difference. HERES whats on my mind. is it possible, the intake is so large that the fuel is not atomizing properly due to low vacuum? thanks Dave

opps: double post

What is your combination? make sure the idle circuit air bleeds are open, what is the timing set at? you may need to crack the secondary throttle blades open alittle or drill holes in the primary blades. Sounds like the engine is starving for air at idle. Your mixture screws should end up at about 1&1/4 turns out. How does it run at part throttle? Is the fuel level set too high-this can also cause a rich condition. Hope you figure it out. :cool:

its a 525hp 383. big hydoroller. the fuel level is right under the plugs. really dont know what basic timing is but 36 total. if it is related to the secondary blades how much do i open them? will this cause a diesling problem? thanks guys Dave

cracking the secondaries alittle won't cause the runon effect unless the plugs are too hot. Experiment with it. I usually pull the carb and remove the set/stop screw then install it from the topside of the carb so you can adjust it while it's on the car. If you're only out 3/4 of a turn with the mixture screws it means you're way rich.

qanother thing to look at is if your accelerator nozzles are dripping fuel at idle, I've seen this happen with nasty cams that actually shake the engine so bad that fuel dribbles out the nozzles. give the accelerator pump arm a c hair of clearance

i checked for a fuel drip Bob and did not see one. also i have noticed 2 times at getting into farely heavy there is a bit of a stumble (very minor) im thinking maybe i need to go to a 4.5 valve. Dave

I'm sure the stumble is powervalve or pump shot related but make sure you clean up your idle before any other tuning. After the idle is cleaned up the engine will be more crisp and receptive to an increase in fuel.

It sure sounds like you have an idle circuit issue to me. Once you clean up the idle circuit you will see a huge clean up of the plugs. I had the same issue with my Demon carbuerator and their tech people had me drill out the idle air bleeds to lean out the idle circuit. Now, I had never worked on a carb before for anything more than an idle speed screw adjustment and they clearly explained exactly what I needed to do right down to the drill bit sizes to buy and how to use a hand held bit driver to ream the bleeds. It worked perfectly and reacted EXACTLY as they said it would.

Bottom line is now I am not at all nerveous about pulling a carb apart. You seem to be comfortable with taking yours apart so perhaps the Holley tech people can give you a step by step process to lean out the idle circuit?

Sheetmetal,

If you can find a copy of Super tuning and modifiying Holley Carburators, by Dave Emanuel, He describes that problem completely. He also gives the reason that it is happening and better yet he gives a solution.

His answer is that the hp is designed for a race engine that requires lots of fuel at idle to overcome a large cam. The best fix according to Dave is to insert a 10-15 thou piece of stainless steel wire in the idle ports. He shows where with great pictures. This will lean the air/fuel mixture that the idle screws control.He also suggests that Holley engineers really don't like people to drill airbleeds as it delays the onset of the primary circut and could possibly cause a lean spot just off idle.

good luck

Danb