340 hesitation/ slow stumble with light pedal input

[Cudajason]

I advanced the timing again, closer to 18 initial timing, all the hesitation/ stumble is completely gone now!
idles around 900 nicely. and no ping at WOT. starts and restarts hot or cold very nicely.
I noticed that while the driver side IMS screw on the carb effects the idle speed and quality the passenger side IMS screw seems to do nothing. closed all the way or backed out 4 turns makes no change. 
suggestions to correct this?
and still surprised that with both IMS's all the way in the engine idles just fine... thoughts?

Back off the idle speed screw on the throttle plate a bit...sounds like it is open to far.

[culp71]

I turned the idle speed stop screw on the throttle plate until engine idled down close to 450, I then turned the pass side IMS screw in then out, no change.
I turned car off, removed the pass side IMS completely, then used pneumatic air gun and shot a few bursts into the hole, cleaned the IMS screw tip, (it was spotless) as recommended in Edelbrock's "IMS screw does nothing troubleshooting section" I then put the IMS back in, started car and still no difference.
only if I remove this screw while its running does it make any difference, very little a little richer, but still maintains idle. I then turn it all the way in and no change.
again driver side IMS screw reacts how it should, but I'm still very surprised that engine will run with both turned all the way in, currently I backed both off to about 1/2 turn out from seated.

also Edelbrock manual says "turn the IMS in as far as possible to get the highest rpm at idle". nothing else, however in Edelbrock's video they say turn the IMS in as far as possible to get the highest rpm at idle, then back off about 1/4 turn.

which is it?

and what could cause one side IMS to do nothing?

[kawahonda]

You will need to rebuild the carb in order to get both IMS screws to do their job.

If one side isn't doing anything and if the carb is rebuilt, check to see if both IMS screws are relatively in the same position. You can have one side way in/out further than the other side, therefor keeping the one side from doing much.

I do not do the "back off 1/2 turn" bullcrap. Set them to where they have the highest idle/vacuum, and leave them alone. Edelbrocks have a slightly improved metering system than Carter however.

Personally, I prefer to use a vacuum gauge for IMS screws since the reading is a little bit more fine.

I would be very curious if your carb is WAY over jetted/too thin metering rod if you can run the engine with both fully in, or even a 1/2 turn out. That may clue you in to a jetting issue. Have the rods and jets been changed? I wouldn't imagine a 1406 would be that far off from factory settings on a 340 where you could crank both IMS screws all the way in and have a running engine.