I'm certainly not an expert, but you might consider the following.

Torque is higher at higher rpms, lowest at idle.

Anything that reduces the engine's power (valves in need of adjustment,
for example) will reduce torque across the engine's range of rpms. At
high rpms, you may have enough spare torque to propel the boat normally.

When you accelerate, or shift to reverse (assuming you're moving forward
at the time) you may find the torque is reduced enough to stall the
engine, or at least to make it sluggish.

Of course if you are at rest and shift into reverse, you would
experience the same load as shifting into forward. Well, the prop would
be running "backwards" and the transmission might use a different ratio
for reverse. Maybe there is a difference.

Bottom line: I would look at those things likely to reduce the overall
performance of the engine. Regarding injectors, I can at least imagine a
change in injector performance at different speeds. Someone more
knowledgeable than I should be consulted.

Good luck.

Chuck

Jay Mott wrote:
Thank you.
The problem has increased over about a hundred hours. What are the
chances that both injectors are fouled equally (idle is smooth)? How
could it run at speed if the fuel filter is clogged? I don't think it
has an air filter.

Tom