Dad bought a '74 merc 9.8 (SN- 3924668) which starts OK and runs OK at low
speed, but will only run at high speed part of the time. When you open up
the throttle to wide open, the engine runs rough at about 30% power for a
while, then will all the sudden kick into full power. It may take only a
minute, or it may take 15 minutes to get to this point. Once at full
power, it runs fine until you reduce the power back to idle then the problem
occurs again. The problem is more pronounced when the engine is warm. It
acts like it's only running on one cylinder when at high speeds. Has anyone
experienced similar issues or have a recommended checklist to trouble shoot
this problem.

I'm sure it's either fuel or electrical, so to start, I thought I would
check to make sure the fuel system is clean and there is a good ground for
all electrical / ignition parts. I'm not very familiar with outboards
though....