Idling Issues - Holley Carburetor

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larsry

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I have a 340 motor that had an Edelbrock 750 cfm performer carburetor and the car idled at about 900 to 1,000 rpm. I switched it out for a Holley 750 cfm street hp carburetor and am unable to get the car to idle properly. When first installed, the motor started and died within seconds. I adjusted the idle mixture needles which enabled the motor to run without dying. However, I am unable to get the idle speed (with the curb idle speed screw backed all the way out) under 2000 rpm.

Anyone know what the problem could be? Appreciate your input.
 
Vacuum leak or throttle cable holding carb open. Would be my first guesses.

Check cable first. If not it, spray carb cleaner around carb and listen for an idle change. Most likely to be a vacuum line left unplugged, or carb base gasket not on correctly.
 
Is the cold idle speed off when you are trying to adjust the warm idle?
 
Yeah, make sure the choke is off, the secondaries are fully closed, the pcv is functioning, the brake booster hose is on or plugged,the primary is closing to the curb,the vacuum advance is not engaged, and the T-port is synced.
If you still have problems, it may be a a vacuum leak, or a low-speed carb issue;assuming the timing was not changed during the install.
 
Very likely, the secondary stop screw is waaay out of adjustment. Set it so that the secondary transition slots are juuuust visible with the secondaries closed, and then reset the primary idle mixture and curb idle adjustments again. It may take a couple of adjustments of this secondary stop screw to get things right.

And if you have a stock Mopar PCV, that can be part of the problem; some of them let in too much air at the higher idle vacuum levels. What cam do you have, and what is your normal idle vacuum level? (But since the old carb idled OK, this my not be any issue.)
 
^^^^ This. Can you tell if the secondary throttle plates are even close to seating closed or not. The stop screw takes a tiny screw driver and is very tough to set on the car unless the carb is on a 2" spacer.
 
Thanks for the tips everyone. I will attempt to go through the troubleshooting process this weekend.
 
mguner said:
^^^^ This. Can you tell if the secondary throttle plates are even close to seating closed or not. The stop screw takes a tiny screw driver and is very tough to set on the car unless the carb is on a 2" spacer.
Click to expand...
I can pull the carb, adjust the screw, and re-install in about 5 minutes. And it let's me see exactly where the throttles are in relation to the t-slots. I have read that you can invert the stop screw so it is adjustable from the top. Or I may put in a hex set screw at some point to make it easier to adjust from the bottom with the carb on..
 
nm9stheham said:
I can pull the carb, adjust the screw, and re-install in about 5 minutes. And it let's me see exactly where the throttles are in relation to the t-slots. I have read that you can invert the stop screw so it is adjustable from the top. Or I may put in a hex set screw at some point to make it easier to adjust from the bottom with the carb on..
Click to expand...

I did that during the tuning phase. Trouble is the arm comes down and slaps the screwdriver slot, so after the tune was in, I put it back. On the AG is is possible to adjust it from the bottom with a sawed off mini-driver, but it sure is a PITA.
 
does the carb have holes drilled in the throttle plates?
does the carb have the screw down the hole for the air cleaner stud like the demons??

i would adjust the throttle plates so that the primary blades make the transfer slot look square, .020" showing, then adjust the seoncdaries so that you can just barely see some light if you hold the carb up and look down the venturi's.
 
also, i wouldn't even begin to worry about the idle feed restrictor until you can get it to idle, that won't affect idle speed.
 
cookietruck said:
also, i wouldn't even begin to worry about the idle feed restrictor until you can get it to idle, that won't affect idle speed.
Click to expand...

It will if the damn thing is too small to flow the required fuel amount TO idle. Wheel those idle feed screws 3 turns out and see if that does it.
 
I'd disconnect and plug everything vacuum related on the carb. Isolate the intake tract. If the idle comes around, reattach each vacuum hose one at a time and see if idle changes. Look for the offender.

Is the large PCV plugged? Seen that one a lot!

Make sure none of the linkages are hanging up on the intake manifold. Look real close at the pass side secondary throttle shaft end.
 
Did you the adapter arm from Holley that puts the throttle cable ball in the correct position?
 
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