The problem with old CB750's is that you can slide a feeler gauge betwixt valve and rocker and the clearance might look correct, but if the hard facing on the rocker or the valve is pitted or otherwise damaged, you could well have a real clearance of 3 or 4 times the reading that you're getting from your gauge.
It could be a buggared spring too, (it may have just lost tension after sitting compressed for many years) but if you've done the adjustment correctly (and a lot of folks don't) then the only way to properly diagnose it is to pull the engine. Cheers, Terry.
