I have a mid-70's 9.8 hp merc. It's 50:1 as stated above.

Definitely flush the motor. Big problems with corosion inside the power-head as well. There are several very small passages in the powerhead where water needs to pass to keep the engine cool. They corrode shut quickly after salt water use. I have seen this problem on the only two mid-70s merc 9.8 powerheads I've ever looked at including my own. A real pain in the neck to clean. If the motor stops pissing water at higher speeds for no apparent reason this may well be the cause.