with it running spray a little wd-40 around the base of the carb.if idle changes you have a vacuum leak on the base gasket and this could be a case of the wrong gasket.years ago i bought a 2x4 offy intake for my 455 olds.had a pair of afb's.guy said he could never get it to idle.took carbs off and he had a q-jet gasket under the back carb, which left a gap on the rear butterfly.as for the mixture screws.they used to be on the front.and easy way to suck the crap out of em if it has a choke flap.rev the engine up to say about 2000 and then hold the flap shut until the engine about dies.90% of the time this will suck out whatever is in the circuit