140 HP dead spot

[copter3]

Compression 120 lb. on all four cyl, strong spark on all 4 cyl, ign timing very close to specs, 16 degrees at 4100 rpm, carbs removed, all passages tested for obstructions, no intake leaks found( used propane), engine idles fine, accelerates to 2200 and runs great, howerver acceleration to wot needs some jockeying of the throttle, once at 4000 plus rpm it is a great running motor. any ideas?

[boats.net]

Check sync on carbs and linkage.