SBC will not idle, need ideas

[Dirtman29]

Only ran 3 nites with it, and on the 2nd nite it started dying out if the RPMs dropped under 1700. Looked it all over, rechecked the lifters, readjusted the carb. Makes good power on the track but I have to constantly have my foot on it to keep it running. Or set the idle @ 2K. Here's what I have:

406 SBC, stock 2 bolt bottom end, dome relief (recessed in the center) .030 pistons (approx. 10-1 motor I was told)
Comp solid cam #12-512-5, lash set @ .018 int./ .020 exh.
Double hump heads, mild porting, 2.02/1.60 valves (64cc)
Timing @ 33 deg, new HEI dist from KMJ performance.
Holley 4412 carb, 72 jets, on an old Edelbrock (TM1) alum. intake, 1" wood spacer
Holley 110 gph fuel pump (new)
Currently running 50/50 shell V-power & 111 race fuel.

Easily turns 6600 RPMs, starts right back up when it does die out, temp @ 180-190 every nite (so far). Just refuses to idle in the staging area, have to constantly rev it. I see no leaks anywhere on the motor, checked for intake suckin' in air & found nothing there. Any help greatly appreciated.

[Dirtrunner35]

Take a bottle of propane with a hose on it and when the car wants to die, turn it on and put the hose in the carb and see what happens. Does the idle go up ?

[let-r-eat]

Needle and seat would be my first guess but could be lots of things.

[Dirtman29]

Take a bottle of propane with a hose on it and when the car wants to die, turn it on and put the hose in the carb and see what happens. Does the idle go up ?

If I try that, will my face still be intact!?

Never heard of this method for diagnosing a problem, what would I be looking for it to do?

[Chaps46]

Check the power valve or swap it out. It's probably blown. ie way too rich at idle but plenty of fuel to make power.

[parrot69777]

Sound to me like your idle mixture screws are not right. Need to turn them out some.

[joedoozer]

I had the same cam in an engine with a 4 barrel vacuum secondary carb that was doing the same thing. The motor wanted more air at idle that the carb could give it. If you opened the primaries up enough to let the motor breath, you uncovered the idle transfer slot too much and idled at 2k. I fixed mine by opening the secondaries and closing the primaries back to where they should be.

I am not familiar with 2 barrels. But if you take the carb off and flip it over. Use a feeler gauge to set the idle transfer slot exposure to what it should be (.020 on my carb). Like I said I have never run a 2 barrel. Could you drill a hole in the throttle plates to let more air flow? I just sounds a lot like the problem I had. Completely different carb, but same symptoms.

Also if you have the throttle plates to far open at idle (exposing too much of the transfer slot), you can't do the shade tree test for the power valve. Turning the idle mixture screws in all the way to see if it shuts off. In my case the engine still ran. The power valve was good, but it was pulling so much fuel through the transfer slot that it would stay running with the mixture screws turned in.

In fact that could be a test you could try. If you know the power valve is good. Run the screws in and see if it stays running. That will prove to you that you are no longer in the idle circuit.

[Dirtman29]

Thanks for the replies, I am thinking its a carb issue, was a used carb & I know nothing about it other than its got 72 jets. If I turn the screw$ in, the motor does want to die out. They are at 2 1/2 turns out now. At the track this past saturday, I let it warm up for a few mins before hotlaps & it did idle then, but would not idle the rest of the nite. Giong out now to mess with it again, hoping to race it friday.

[Dirtrunner35]

Your face will be fine, LOL. If the rpm's go up with the propane it's too lean, the idle mixture screws should be out 1- 1 1/2 turn that's it. Try a different carb.

Propane is great for finding intake leaks.

isn't 72 jets really small ?

[Dirtman29]

Your face will be fine, LOL. If the rpm's go up with the propane it's too lean, the idle mixture screws should be out 1- 1 1/2 turn that's it. Try a different carb.

Propane is great for finding intake leaks.

isn't 72 jets really small ?

Had my brother look at it & he readjusted the carb, it was set way too rich. Explains why I used 7 1/2 gallons of gas in 25 race laps! He also suggested bigger jet size, 76 or 78s. Also had 2 burned plug wires arcing on the headers that I didnt notice til today, put a new set on and it runs & idles fine now. Thanks all for the help.

[sj valley dave]

If it basically a stock 4412 500 2bbl, 72's might be close...just look at your plugs, your enegine will tell you if it wants more fuel...Nice light tan color..