I'm resurrecting this thread as I am still having this problem. Things I've done: cleaned and rebuilt all carbs with BRP repair kits. Cleaned throttle bodies, verified all passages into and out of calibration pockets are clean. Set idle mixture to basic setting of 3 1/2 turns out per factory manual. Rebuilt fuel pump side of VRO pump. Replaced fuel filter and water seperator filter. Vacuum tested fuel lines to verify no air leaks. Link and sync performed per manual. Set timing using Joe Reeves method. New spark plugs, verified good 7/16 spark, all cyls. Compression is 140, 145, 140, 140 cold. Fuel is fresh. The engine starts easy and idles OK. It will run well up to about 3400 rpm. When I try to throttle up slowly from there, it shudders, shakes, and misses badly, doesn't stall. Moving the throttle up more it clears out and runs fine above 4000 up to WOT 5600 rpm. It will cruise all day at 4200 - 4400 rpm, nice and smooth, but if I try to back it down below 4000 the shuddering and shaking starts down to below 3400, then it clears up again down to idle speed. The throttle plates are just open about 5 - 10 degrees from closed when this starts. Fuel is being drawn up from the high speed jets. At cruise, the throttle plates are open about 1/3 of the way. I'm stumped as to why this is at such a narrow rpm range and welcome all suggestions. Thanks in advance for your help.