I put 2 new spark plugs in my outboard, and now the top speed is about 1/2 of what it was. The gap is set correctly, and it's a 1990 Johnson. Does this make any sense? It's starting fine and idling fine. It's the same new gas I used 3 days before. Thanks for any advice.

Make sure the spark plug wires are pressed on properly. If they are, then put the old plugs back in, and see how it runs. If it runs OK, the problem might be a bum plug. It's not common, but it can happen.

When it's running at half speed is it running on one or two cylinders? If it's running on two cylinders, check the shift and throttle linkages. There is a limiter in the shift linkage that will not allow full throttle operation unless the motor is in gear. It's possible that linkage needs adjusting to allow the throttle to open fully.

Thank you. I feel very foolish, but the problem was I didn't put one of the spark plug wires back on. Foolish AND senile maybe!!

Don't feel foolish. Be happy that it was a simple and inexpensive (free) fix.

Oh yeah !!