i have a holley 750 HP model chokeless double pumper. it is running insanly rich at idle. it is smoking from too much gas. i have blocked off both power valves and still no difference. my cam is a comp cams 292H magnum grind with 501/501 lift and 244/244 duration at .050". it makes almost no vacuum at idle which is why i plugged the power valves. the carb has 4 corner idle screws. i also changed jets and seem to have no affect. any help.

Main jets won't affect idle mixture - at least, they shouldn't.

I'm assuming adjusting the idle mixture screws doesn't have any effect. What may be happening is the throttle blades are open too far, either primary or secondary, or both, which is defeating the idle circuit. The throttle blades shouldn't be open any more than enough to expose the transition slot about the same as their width (you have to take the carb off and turn it upside down to see this).

If you can't get it to idle with the blades closed that much, you may need to drill a small hole in each blade to let it get enough air w/o opening beyond the transition slot.

Another possibility is you need to change the idle air bleeds.

Check to make sure your blades are closing all the way and not sticking open. I think your cam isn't so big that it should be meesing up you mixture. I have a bigger cam than .500 on a 830 Holley race carb and had some idle rich problems that turned out to be the blades not closing all the way. Once you set your idle with the idle screw, go to the four corner screws. Turn them in until the idle stumbles a little (go slow as it take a couple seconds for the adjustments to have an effect). When the car stumbles turn the screws out about 3/4ths of a turn and repeat than process at each corner. The tendancy is to open up the blades so much that the carb comes off the idle circut causing the motor to run rich. Your car should idle at 800 with NO problems. Once the car is tuned right you should notice an improvement in the vacuum output at idle.

i bought the carb used from a guy running it on a race car with a huge solid cam so it already has small holes in the throttle plates. how far open should the secondary plates be? the primary plates are adjusted with the idle speed screw right? if they are then won't idle speed suffer? if not how?
right now i have the idle screws turned out 2 turns. my carb book says thats a good place to start and work from. this where i should start turning them in?

Just to be safe back them out 1 turn and start the process there. Secondary plates should be closed and as said above the Primary should be somewhere "in" the transition slot. Actually there is a specific location in the slot they should be but it's to hard to explain. Start by adjusting the idle circut if that doesn't help you can call Holley they are VERY helpful and can verbally talk you through it pretty well. Have your cam specs handy though. Also check to make sure you have enough return spring as sometimes the spring isn't strong enough to seat the blade if there is binding. Cycle it a couple times and let the blades close slowly to see if they return to normal. It take some time to get it right but it's worth the effort.

You must run a lot more initial advance to get this motor to idle right. like 32 to 36degrees at idle.

Why...... because the big overlap of the 292magnum can causes
so much EGR effect at idle that the fuel burns too slow and enters the exhaust at idle with stock ignition timing. the motor will tend to run hot at idle too.

getting the ignition advance right will cure this.

Remove the distributor cap and rotor. remove the advance weights and springs. Tie wrap the advance mechanism fully advanced and locked out. Install the tie wraps so the rotor can be reinstalled.

restart the motor and re time it to 32-36deg at idle.
The ignition timing will not advance now with rpm as it is locked.
Without vac advance... now you can connect vacuum advance to ported vacuum.

Re-set the idle mixture screws and idle speed after it warms up.
The idle mixture screws should be responsive now.

reinstall 3.5" power valves and stock jetting (70 pri 80sec)
(use a #76 sec jet of there is a sec power valve)


if the idle feed restrictions were enlarged or the idle air bleeds were modified for the previous race motor you'll either have to replace the metering blocks or reinstall the right size orifices in each.

Be sure you have a operational PCV system hooked up to the manifold or base of the carb.
The small air leak of the PCV allows extra needed air into the motor to allow sufficiant idle speed. Allows correct throttle blade reference when use in addition to the locked out ignition timing on motors with big cams.

Now remove the carb and look at the bottom of the carb at the throttle blades. the idle transfer slot should be barely exposed
by the throttle blades.(.030-.040") the primary and secondary blades should be evenly opened. if not,,, readjust them.
Probabily won't need the holes in the throttle blades for correct idle if you follow my tuneup.

fbird, I've seen you post about the timing lockout before and while I agree that it can help in some cases for both street and race machines I have to wonder why you don't also state that they'll need an ingition cutout to start the car hot. With that much initial timing the engine will not want to turn over when it's hot. I've seen this spit out some high dollar gear reduction starters after a short time of struggling.

I have seen most cars respond by improving idle when the initial timing is increased to around 22 to 25* so a total lockout might not be necessary. I'd still suggest the ign cutout switch for starting with anything above about 18 initial. Starters will last much longer.

You can try screwing the idle mix screws all the way in to see if the carb is running on the idle circuits or the transition slot. If you still idle rich with them screwed in then you're on the transition circuits and will need to readjust the throttle blades (probably on both sides).

When the engine is hot just hold the throttle open the start.
Gets rid if most of the hard cranking.
Works suprisingly well.

If you want after you've tryed the easy to do timing lock out and
have the motor running a lot nicer, you can have the distributor modified to have a very short timing curve with say 25/28 deg at idle and 32 to 36 total like 305SBC suggested.

But the locked out timing is not that hard to live with, with a big cam like the 292magnum.

The ignition cutout switch sounds like a good idea.

I run 20 degrees initial timing and I used to run 12 initial and have a WAY bigger cam and it didn't seem to make my car run rich at idle with the "normal" timing in it. MSD however did help my car greatly with the rich idle condition.

Sorry 'bout your luck.....just kidding...

25 to 36 deg at idle is "normal timing" for a motor with a big cam like the 292magnum.

You have to light the fuel early at idle because the big cam's overlap makes
it burn slower at idle. Try it.

well guys thanks for the help. i got a chance to mess with the car on saturday. i put the 3.5 power valve back in and jetted it to 70/80 like suggested. i closed the secondary throttle plates. the car now idles at about 750-850 it has 4" of vacuum at idle but still smokes black from being rich at idle. at highway speeds it drives fine except sometimes when starting to pull a hill and light throttle it backfires but thats the only time. i dont have an advance timing light so i couldnt change anything there. i am using an accel HEI distributor so i have some different springs i could try, any ideas. would the sparkplugs have any affect on things. im not for sure which plugs to run.

Do you have MSD? That could solve a lot of you problems and you can probably pick up a 6AL cheap from a swap meet.

Also how are you operating a vacuum advance with that little bit of vacuum. I had to switch over to a MSD mech. advance dist. and ALL my idle rich probs were solved. I'd start looking to your ignition now.

Mine doesn't seem to run too rich at idle... I'm running the 294S magnum cam with 17* base timing. It just leaves a bit of a mark on the ground after a while but I do have the idle mixture screws set a bit rich.

You don't need a advance timing light to set the timing light once you lock out the mechanical advance on the HEI as the timing setting will be the same at idle as at high rpm.
Just reset the timing to 32/36 deg at idle. Then reattach the vacuum advance.

The car is backfireing as you roll into the throttle
because of a lack of ignition timing advance at low rpm.

He will probably need a dial-back timing light in order to read anything over about 18* advance. I think Autozone will let you use theirs while you're in their parking lot.

Or you could mark your balancer at 32-34 and 36degrees BTC by calculating the mark distance from O

Its easy just follow this formula and use a piece of masking tape to measure out the distance on the balance.

balancer diameter /2 X 3.1416 X 2 = Balancer circumference (inches)

Circumference / 360 X # of deg of advance = mark distance on balancer in inches from 0 TDC.

eg: to mark an 8 inch balancer at 36degrees

8/2 X 3.1416 X 2 /360 X 36 = 2.51" from TDC

mark the balancer at 2.51" from 0 TDC as 36deg

Now you can set the total timing without an advance timing light.

or he could just buy some timing tape for about $5 at a local speed shop to mark out timing.