Being a used engine, if it was in a crash or had a prop strike at high throttle the crankshaft could be bent a little. You would need a dial indicator to check that and also check the prop hub as that could have been machined a little bit out of round.
The OP's reference to a "cock and load choke" I believe is a cock and lock choke. The spring loaded type that loads a spring when you set the choke on and after the engine is primed and fires you turn the choke off by advancing the throttle stick a bit and the choke snaps open.