Thank you, everyone! The problem was a plugged vent- I got it cleared
last night and ran it smoothly for 15 minutes.
Mike

wrote:

Hi everyone,
With the wealth of knowledge and experience here, I have a problem that
needs your insight.

My (new to me) Yamaha 9.9 HP outboard (built year 2000) usually runs
fine at lower RPM, but when I ramp up over 35% of max speed the engine
will invariably run fine for 20-40 seconds and then cough, sputter, slow
for a few seconds and then speed back up. The faster the RPM, the more
frequent the problem.

Sometimes pumping the sqeeze bulb on the fuel line will solve the
problem for a short while, but not always.

I immediately suspected a fuel line problem, but the filters are all
clean (in engine and in gas tank), the hose seems to be clear, and just
to make sure I also changed the spark plugs and oil.

The only think I can think of is that it could be sucking air somehow.

Any suggestions would be extremely welcome!

Thanks, and fair winds,
Mike