I have a Briggs Twin II (422707 1214 01) in a Turf-Trac 1848 (MTD 140-848-000) that will only run at 3600 RPM. The throttle lever does not adjust the speed, but it is not over-speeding. It starts fine and will slow down if choke is applied. Choke & throttle cables are not crossed. The throttle cable does move all of the linkages, but possibly less than desired (full cable movement is 3/4" vs 1 3/8"), but speed does not change over that range. I've done the throttle cable set up as per the engine manual (throttle at full & governor control swivel pegged). I've done the governor static adjustment (lever & shaft fully counter-clockwise). I just can't get it to slow down any.
Feel free to assume the PO messed with the entire throttle system. He liked to "make thing work" as opposed to "make thing work correctly".
Engine not running - throttle at slow Engine running - throttle at slow
Engine not running - throttle at fast Engine not running - throttle at fast