I just purchased a 15' fisshing boat with Johnson 48 HP outboard. The
thing sat for a couple of years. I drained the gas and replaced it.
It starts and runs just fine (a little rough at idle, but acceptable).
When I rev the idle throttle, the engine revs to high RPMs as it
should. In the water, when I put it in reverse and nail it, it revs
to high RPMs. Of course it does not go very fast in reverse, but the
engine is revving properly.

When I go forward (full throttle) the engine only revs to about half
power. I can only go about 7-8 mph at full throttle. But it sounds
like it is running ok. I took off the engine cover and observed that
when doing this, the little arm that swings when throttle opens is
swinging the appropriate distance, same as when I rev the idle
throttle.

What gives?