Loss of high end RPM

[hoosierdaddymike]

Have a 1987 PRO50LH on a small bass boat, has run perfectly since I got it last season, ran perfectly this spring until a few days ago. With no warning, it won't plane out, sounds like the throttle stops progressing at about 1/3. Running with cover off you can see the throttle mechanism working as normal, but no increase in engine RPM the last 2/3 or so of its travel. One note: the 20 gallon tank was run nearly dry, put a few gallons in, made no difference. I cleaned the fuel filter (was not fouled) and drained the carb bowls (nothing unusual there). It runs smooth as ever, idle, reverse, forward and through what acceleration there is, just lacks the high end throttle Any tips or suggestions?

 

[99yam40]

compression, and spark an all cylinders would be where I started
If that was good then spark timing, make sure it advances as it should

if all good then fuel problem