Help on adjusting idle on 340 Six Pack

Just received my restored 3 2-barrel carburetors back from Holly. Carburetors are installed on rebuilt but bone stock 340. Engine is being run on a test stand prior to detailing and installation into my AAR. The issue I'm having is as follows: with the front carb throttles completely closed, the lowest I can get the engine to idle is 900 rpm. Not that I need it much lower, but it seems like I should be able to have it idle at 600 rpm if I wanted it to or idle so low it would die by continuing to back out the idle set screw?

Specs: These carbs are original to the car and are virtually new out of the box after a complete restoration by Holley's Custom Shop. Center carb is a 4792 (auto), timing is set at 6 - 7 BTC, Air/Fuel ratio is 12.5 - 12.9 to 1,vacuum is a steady 15 inches, power brake nipple and other vacuum ports are plugged, also, the idle mixture screws on the idle circuit block can be screwed in all the way with no apparent effect on engine rpm.

I need to trouble shoot this problem and I am hoping to hear from a "six pack expert" on where to start and where to go with it.

Are you certain that throttle butterflies on any of the carbs are not hanging up in a bore? No gasket leaks at intake or carb mating to intake, causing vacuum leak,,,fast idle is often a sign, particularly when mixture screws have no effect. Also if throttle butterfly is hanging up in bore you are likely in transition circuit which would cause ineffective mixture screws and fast idle.

How are mixture screw set in outboard carbs?

15 inch vacuum sounds ok, perhaps a little low for 900 rpm, depending on camshaft. Again possible vacuum leak.

Is 6 btc normal for this engine? Sounds a bit low,,,,again causing you to adjust throttle butterflies to be off idle circuit and on transition circuit,,,making all bets to be off

Just throwing thoughts agn the wall. I have six pack on a big block. Runs very well.

what cam? Try bumping the initial up to 15 or 18 (I'm running 18 on a bone stock 83 318) & see if that will let you slow it down.

RapidRobert - 6 BTC is a bit advanced actually, the book calls for 2 1/2 BTC. The cam is a stock cam as defined in the specifications section of the service manual for a 340. If I advance the timing more, the engine will run faster not slower. I can reduce the idle by retarding the timing but that doesn't make the engine "happy".

Sxrrnr - I haven't really inspected the butterflies closely to determine if they are hanging up...they very well may be. I will look for any "hangup" by the butterflies as you suggest and inspect the "transition slots" to be sure they look correct as well.

Probably not a big deal since the carbs have been completely restored and all worn parts replaced with new parts. I assume it is something minor that I am overlooking at this point.

I appreciate the feedback.

sounds like a vacuum leak or the throttle blades are hanging open a smidge on one of the carbs.

should definitely be able to kill the motor with the idle mix screws.

can you kill the motor by covering the main carb opening with your hand??

Actually what I was thinking was to bump the timing up which yes will speed it up THEN slow the idle with the idle speed screws. then recheck the timing & set it back to 15 or 18 if it is (now) below that after you have (1) bumped the timing up and (2) slowed the idle speed. I wouldn't be afraid of 15 or 18 & yes that'll demand that the slots get shortened but one thing at a time & it might solve the idle speed prob. Are you on ported or manifold?. keep us posted.

My 6 pack (mechanical type) idled better when it was lowered to 14 or so from 18/20. If I squared my transition slot on all three carbs it would idle to high, if I closed the outers more it would not run nice and smooth. When I squared all the carb slots up and dropped the initial it ran much nicer and I think the more/ balanced air flow made a big difference, mind you I have 195/200psi cranking presure so I don't think I needed a lot of initial with a efficient comp ratio.

Book timing has emissions related considerations. If you don't have to pass emissions, throw the book numbers in the garbage. In a nutshell, they SUCK for a performance set up!

More initial timing is in order. Fix the distributor mechanical advance (tons of how to on the web) and it will run better.

Stock 318's get at least 10* initial if I work on them. 340's with a stock cam run nice with 14-18. Bigger cam, usually more initial timing.

You definately need all the air to supply the engine from the center carb. If the mixture screws are having no effect on idle, you are correct, the blades are closed all the way.

Air is coming in from somewhere else. I'd block the air horn on the end carbs and if that has no effect, start looking for a vacuum leak.