mike fuge

This 12A engine,Nikki 4 barrel was kitted 6 months ago and ran great,now it wants to sputter/shutter until it gets to 3500rpm then it runs great.I'ck'd fuel pressure & flow...ok.I ran propane into intake ,makes it run -much better.Accelerator pump squirts good and strong.Float levels both right on the money.Spark plugs look great!!I've looked at the primary and secondary ignition patterns...MS & KV look satisfactory.Emmissions are passable for albq. nm...vaccum is a steady 15'hg and the vaccum neddle goes to ZERO when the throttle is punched to the floor (no exhaust-restriction).The emmisions have not been bypassed.The secondaries are not mechanical,they are stock.The "t-bar"is in the correct positionon the linkage in aft portion of the carb.OK...I'm looking at the butterfly under the carb,The linkage is serviceable,but how can I check this out?My informaton comes from Haynes/mitchell/IATN/CHILTON,but most of it is sketchy....so if any of you razzle dazzle wizards can guide me through rotory dynamics I would be greatful....