Rough idle Carter AVS

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Longgone

Longgone

John/68 Barracuda & Dart
Joined
Jul 12, 2005
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Location
Newport News, Virginia
The idle on my stock `68 340 is rough and I think it`s the Carter AVS carburetor. Timing is at 16 initial and 18 mechanical w/electronic ignition. The idle mixture screw seems to work in reverse increasing rpm as you screw it in and removing the air cleaner also increases rpm. I`ve checked for vacuum leaks and haven`t found any. The car surges when driving normal too. Under WOT the car runs super. Please, any help?
 
Sounds lean, reguardless, is this a stock carb on a stock engine?
 
Vacuum leak, To much vacuum advance and or lean in cruise from the metering rods. A swap to a smaller rod will increase fuel flow at cruise.
 
Why would the rpm increase as I turn in the idle mixture screw? I have 16* of advance, that must be enough advance to allow the carburetor to operate in the idle circuit at 800 rpm?
 
The stock jetting is actually rich, or should be rich as that is how the factory set them up. When you close off the idle or down the idle screws, the mix leans out and air speed picks up since theres less fuel being mixed in with the air to slow it down.
An engine can run on very lean ratios.
The engine could also be pulling fuel out of the carbs main curcuit.
 
Longgone said:
Why would the rpm increase as I turn in the idle mixture screw? I have 16* of advance, that must be enough advance to allow the carburetor to operate in the idle circuit at 800 rpm?
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Well I think its too rich if the rpms go up as the car gets leaner. Maybe the rod or rods are suck up high. They should go down when the motor is idling. Take the rod covers off and make sure they can be pushed down easy, if they were stuck at first, that might have been it.
 
I `ve checked out the rods and even swapped them out along with the springs but with no change in how it runs.
 
Well then its a vacuum leak somewhere or some dirt clog up part of the idle circuit. Either one of those would give a rough idle but it still would run great WOT.

This is when its nice to have a spare carb you know is good and adjusted for your motor.
 
I`ve checked for vacuum leaks and can`t find a thing, Where do I go from here? The plugs are very clean as though it`s running super lean.
 
I have not had the carb apart. It was just rebuilt by a reputable dealer and is supposedly ready to plug-in and play. Could the intake be leaking on the inside?
 
I thought it just started idling rough, not that you just swap carbs.

If it was idling, running OK and now you can not get a good idle, I would think its the carb.

Float level it could be, along with plug air bleeds or passages.

You do have all the unused vacuum ports plug, right, even the one in back of the carb, if theres one.
 
It`s the original carb, just rebuilt.......didn`t run all that great before rebuilding but this is a new rebuilt engine now with new rings, bearings, cam, ignition, etc.. The more I read about the carb and tuning ,the more it points to a vacuum leak somewhere. I`m almost ready to change the manifold and carb gaskets out just to rule that out. The car has 13-15 lbs. of vacuum at 850 rpm and I`ve used carb cleaner and checked all the gaps for leaks with no increase in rpm.???
 
Did you check for a vacuum leak at the throttle shafts? Older carbs sometimes have worn shafts/main body housing where the shafts meet the main body and need to be repaired to get a good seal.
 
I went over the intake and carb again looking for vacuum leaks. I found the tiniest of leaks at the right heat crossover area of the intake. I got the rpm to increase about 40-50 rpm if I sprayed that area heavily with carb cleaner. So what did I do but change out the intake and carburetor gaskets. After $15.00 worth of gaskets/silicone and half the afternoon I`m no closer to getting this idle /surge issue resolved than when I started. No vacuum leaks at all now but still:
1) the car won`t idle steady
2) idle goes from slow to fast if you feather the gas
3) rpm/vacuum increases if you screw in the idle mixture screw to stop
4) car will cut off when put into gear unless you keep a foot on the gas
5) runs great at full throttle
6) surges at idle and cruising
7) engine speed increases dramatically if you partially cover carburetor throat
8) plugs are very clean

Any more ideas, I think it`s running really lean. Re-jet? New metering rods and springs? Where do I go to richen this carb up?
 
Rocket said:
Did you check for a vacuum leak at the throttle shafts? Older carbs sometimes have worn shafts/main body housing where the shafts meet the main body and need to be repaired to get a good seal.
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I did check that with the carb cleaner, all was good.
 
""engine speed increases dramatically if you partially cover carburetor throat""


That right there screams "vacuum leak(s)" to me. Sounds like you're covering most of the bases though.

13-15" of vacuum sounds a bit low to me for a stock-cammed 340. Shouldn't it be more like 17 and up ,or so?
 
Longgone said:
I went over the intake and carb again looking for vacuum leaks. I found the tiniest of leaks at the right heat crossover area of the intake. I got the rpm to increase about 40-50 rpm if I sprayed that area heavily with carb cleaner. So what did I do but change out the intake and carburetor gaskets. After $15.00 worth of gaskets/silicone and half the afternoon I`m no closer to getting this idle /surge issue resolved than when I started. No vacuum leaks at all now but still:
1) the car won`t idle steady
2) idle goes from slow to fast if you feather the gas
3) rpm/vacuum increases if you screw in the idle mixture screw to stop
4) car will cut off when put into gear unless you keep a foot on the gas
5) runs great at full throttle
6) surges at idle and cruising
7) engine speed increases dramatically if you partially cover carburetor throat
8) plugs are very clean

Any more ideas, I think it`s running really lean. Re-jet? New metering rods and springs? Where do I go to richen this carb up?
Click to expand...

#7 tells me right away that your lean. i would step up the rods to the next size and maybe the jets too, but check to see what you have first. rods are the easiest to change out. check the fuel inlet needle and filter. plugs being clean means they are a rusty tan color or white? is your vacuum advance hooked up?
 
The plugs are white with a yellow tint. Vacuum advance is hooked up. Are the rods and springs still available for these carbs? I read somewhere that you can only get kits for the AFB`s?
 
take it down to the speed shop and see if they are the same as the thunder avs ones by eddy.
 
The local Mopar guru looked at the carburetor today and I`m happy to report that he got it dialed in beautifully. The rebuilt carburetor on the car was way lean and after checking the idle jet, he went to the two screws located at the front of the baseplate. The holes in which the screws are located were filled with lead. He removed the lead where he found both screws turned all the way in. He turned them both out about 3 turns and then adjusted the idle mixture screw out about 1 turn. What a huge difference!!! The carb rebuilder had screwed those 2 screws in all the way then leaded over them. They had been the problem the whole time. Now the car idles really smooth, I`m very happy.
 
Now thats the most F$%^* UP thing I ever heard of and unless there was a picture posted to see what he seen, we would have never got that problem.

I can't believe I just read that! Thats insane! Freakin crazy!
 
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