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Mystery rough idle - barely run

TopBanana72

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Background... Freshly rebuilt 318. About 2500 miles on it now - went to Carlisle and Nats in it. No issues. Swapped from 20 year old Holley 4160 to a new Edelbrock AVS2 carb. Worked great, had an issue adjusting choke/fast idle at first, but found problem. Everything good for a few days. But then... Coming off a 60 MPH highway trip to an idling stop, stalled. Can restart OK, but idle is extremely rough now. Shakes the car badly. Car very hard to keep from stalling at low rpm. Here's what I checked:

Timing - OK, (it like about 1 degrees initial)
Idle Mixture - OK, set to highest vacuum (although vacuum reading is now from a high of 19 in the past to reading around 15 now.)
Choke - Functioning properly
Carb Obstruction - no obstruction
Vacuum leak - none that I can detect
Distributor Cap - looks good
Rotor - Looks good
Push rods - OK, tight against rockers
Rockers - OK, rockers are rocking
Compression Test - All cylinders good and near the same number (about 127)
ECU swap - no change

Any ideas? thanks!
 
I
Background... Freshly rebuilt 318. About 2500 miles on it now - went to Carlisle and Nats in it. No issues. Swapped from 20 year old Holley 4160 to a new Edelbrock AVS2 carb. Worked great, had an issue adjusting choke/fast idle at first, but found problem. Everything good for a few days. But then... Coming off a 60 MPH highway trip to an idling stop, stalled. Can restart OK, but idle is extremely rough now. Shakes the car badly. Car very hard to keep from stalling at low rpm. Here's what I checked:

Timing - OK, (it like about 1 degrees initial)
Idle Mixture - OK, set to highest vacuum (although vacuum reading is now from a high of 19 in the past to reading around 15 now.)
Choke - Functioning properly
Carb Obstruction - no obstruction
Vacuum leak - none that I can detect
Distributor Cap - looks good
Rotor - Looks good
Push rods - OK, tight against rockers
Rockers - OK, rockers are rocking
Compression Test - All cylinders good and near the same number (about 127)
ECU swap - no change

Any ideas? thanks!

I meant to say 15 degrees initial
 
Check for burnt plug wires and or a bad or failing coil. Check for flooding. Check for fuel boil.
 
Check for burnt plug wires and or a bad or failing coil. Check for flooding. Check for fuel boil.

Thanks.. .The wires are good. I HAVE seen boiling in the glass fuel filter. And there is no car spacer. I'll try a coil swap, I have a spare.
 
Dirt in fuel got into the carb or the new carb already contained the dirt. Blow out thru air vent in carb and see if idle improves
 
Have you pulled the plugs and what do they look like?
 
Thanks.. .The wires are good. I HAVE seen boiling in the glass fuel filter. And there is no car spacer. I'll try a coil swap, I have a spare.
If for some reason you don't see improvements with suggestions previously stated, try an Edelbrock 9266 heat insulator. Ever since I used one on the 440 it basically eliminated any heat related fuel issues I had compared to the much thinner base gasket.
 
I would pull a plug wire at a time while it's running.
If you have dual exhaust listen to each side.
What style ignition?
 
Dirt in the needle valve or a vacum leak. Check all the intake bolts.
 
Dirt in the needle valve or a vacum leak. Check all the intake bolts.

Intake bolts good. I took the the idle mixtures screws out all together and needles look good. Sprayed air into the holes, replaced screws and re-did idle mixture procedure.
 
I'd say fuel boil is a very good candidate. Its cheap enough to fix and wont break your wallet.
 
If it's strictly at idle/low RPM's and the car seems to run ok at higher RPM's, I'd go with a vacuum leak. A quick check with a vacuum gauge might tell you something.
 
PCV valve in good order?
If it failed it will allow the engine at idle to suck in a lot of air through it instead of a small metered amount, which makes it run very lean.
If in doubt, disconnect and plug the hose and see if it solves the problem.
 
If it's strictly at idle/low RPM's and the car seems to run ok at higher RPM's, I'd go with a vacuum leak. A quick check with a vacuum gauge might tell you something.

It sure does seem like that. I checked for leaks, but I'll have to check again.
 
Did you try your good coil yet? I've had them fail and present the same symptoms.
Grab a .010" brass feeler and check the reluctor wheel to pickup gap inside the distributor too.
 
Did you try your good coil yet? I've had them fail and present the same symptoms.
Grab a .010" brass feeler and check the reluctor wheel to pickup gap inside the distributor too.

will do
 
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