"You need to find out (if
"You need to find out (if you don't aready know) the top rated (full throttle) rpm of that engine, and have a tachometer of course.
Now, with your average load, at full throttle, the rpms should read close to that setting. For example, the top rated full throttle RPM for the two various 1987 OMC 150hp models are (don't know which one you have):
The XP150 = 5500rpm
The Reg 150hp = 5750rpm
You would want your full throttle operating range to be as close to the above settings BUT not to exceed those figures (Piston Damage). Usually with the 5500rpm figure, we strive for a reading of about 5200rpm or 5300rpm.
If you're in the above ballpark at full throttle BUT have the problem you speak of when trolling, the only thing I could recommend is that you install a lower pitch prop just for trolling and make a point of manually not exceeding the top rated rpm OR purchase a four blade prop which would provide a tremendous increase in thrust which might offset the heavy fellow problem.
The big problem is that the engine is really not designed for trolling but we all use them that way. Hope the above info helps out some.
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