anyone know what causes a 4100 to idle rough below 700.it has 20 vacuum and the idle screws answer as i can stall it.its not flooding but something is wrong. ill try a kit but what has me spooked is i tried another carb and it worked the same.when i give it a shot and i let it back to idle it will be smooth for a second or two.

Sounds like a compression check is in order.  You may have one or more valves not sealing like they should.

John - "The Hoosier Hurricane"

Rough idling can can many causes.  You may have an ignition problem.  Check the carb by looking down the barrels with the engine idling.  Do you see any fuel dripping into the intake as it idles?  If you do, the floats are out of adjustment or you have a bad needle and seat. I don't think the idle circuits are clogged as you can stall the engine by turning them in.   You could also have worn parts or something out of adjustment.  Next I would be checking the ignition wires.   Are they old or worn?  Are any touching the engine or arcing?  Check the wire going into the coil.  You can chase that problem for quit a while.  Been there  done that.  Is the engine in tune?  Plugs, points, timing?  All these things can cause rough idling.   Lots of people have bought a new carb and found out later they had a simple ignition problem.  Like John said, valves could be a problem.  Run a compression test.

its a new engine still on the stand.new plugs wires etc.ill do a compression test to be sure.its a 292 with new bore and rebuilt g heads.i dont know if its any use trying yet another carb.

Don't forget to check the distributor cap, points and condenser.   I installed a rebuild distributor a few months ago.  I bought it on line from a reputable parts house.  My car ran fine for a few hours then left me on the side of the road.  I replaced the points and condenser with quality parts and no problems ever since.  Lots of parts for these old engines are coming from China.  Check for vacuum leaks as well.

checked the compression and ok since i had the plugs out anyway i put in another new set autolite.the first carb was flooding and the plugs were dirty.at least an 80% improvment.still a slight tremble but the rings are not broke in so that shound help.some y blocks always have a little shake for whatever reason.dont know if the plugs were bad from the start or not.thanks everyone.

Speaking of while it's tough to determine exactly what's going on here or what you're describing it is in general terms a really bad idea to excessively idle an engine with no load, (ex. a new engine on a stand) a classic error where the owner spends a lot of time futzing around with the carburetor and/or engine timing. Not trying to beat up on you, maybe you know this already, but the cylinders tend to glaze over right away and the rings will never seat.