[Bob Lincoln]

Could be lots of things.
1) Vacuum leak.
2) Incorrect point gap.
3) Incorrect ignition timing.
4) Lean idle mixture at carb.
5) Incorrect idle setting.

Best to do a full tuneup, making sure everything is to spec, including idle mixture, fast and slow idles, choke adjustment, proper function of heat riser valve, no vacuum leaks.

 

[68RT]

Also check for worn timing chain.

 

[AC TC]

Non ported vacum advance.

 

[AHBGuru]

- Fix the choke. It likely needs a new choke thermostat.

- Ditch the points distributor for an electronic distributor. You should be able to find a new (not rebuilt) aftermarket dizzy, go with a '74 318 2bbl application when you shop. The Cardone 843890 would be a good choice, they run about $60 at RA.

- As noted above, check for timing chain slop. If bad, replace with the Melling double-roller set.

- Curb idle mixture, set as lean as possible.

- Hot curb idle speed of 625-650 RPM is fine (1200 for cold fast idle is good).

After all that, if it won't provide a steady idle, get a can of Mopar Combustion Chamber Cleaner (the foaming spray), run the engine on the fast idle cam, and carefully spray down the carb throat until it bogs way down and dies. Let her sit for a few hours (overnight is best), then start her up. It will smoke and stink like a refinery blaze. Once it's warmed up, take it up on the highway and give it a good old fashioned Italian tune up (open her up). That should blow all the snot and boogers out of it.

 

[ImperialCrown]

Is it a slow idle or a rough idle?
Both conditions may be adjusted out, except if the rough idle is due to a misfire.
You want both idle mixture screws at an equal number of turns out at a warm idle. Then turn each an equal amount in until there is a slight lean stumble. Doing it by ear isn't the best way, but it should get you in the ballpark.
Some speed drop in Drive or Reverse is normal. There may be other issues (ignition, engine mechanical) afoot.
Always diagnose first.

 

[chuzz]

If you try what Imperial Crown suggested, a vacuum gauge is extremely helpful in this procedure. I've got one that over 50 years old. It was my fathers. None of my idiot brothers wanted it as they didn't think it was a useful tool !!!

 

[GoManGo0406]

In addition to all of the items mentioned above, if none of those fix the issue, also pay close attention to the advance weight springs inside the distributor. Look for broken/missing spring(s), or a possibly stuck weight that does not freely move. Back in the day it was a pretty common problem to find broken/missing springs especially in the 340 motors before they went electronic ignition.

 

[bach4660]

thank you everyone. Where I am at now, is the VECI says 650 RPM but it won't idle at 650 making it hard to make any adjustments. I 'm now at 850rpm with timing correct, 20hg of vacuum on a steady gauge. Took for road test and ran smooth, no more stalling at traffic lights. So all good, just not the spec'd rpm, but it is 52years old and not my daily driver, so happy for now. Will come back to it when the weather gets better.

 

[ImperialCrown]

If you can't get the idle speed lower without it stalling, try to determine if it is too rich or too lean so that we know which direction we want to go.
If the engine speeds up & smooths out by spraying a small shot of carb cleaner above the carb, it is too lean. This may be from a mixture adjustment or a vacuum leak.
If the engine speeds up & smooths out by pulling off a vacuum hose and getting a suction hiss. The mixture is too rich. Again, this may be a mixture adjustment or a flooding carburetor.
Looking down in the carb, do you see gas dripping from the venturis? The spark plug tips might show signs of carbon fouling if too rich.