90 hp Johnson
First I would try cleaning the carb. It sounds like you may have a main jet that is plugged. The main jet meters your fuel from the 1/2 to full throttle position. That is a cheap and easy fix.
If that doesn't work I would check your fuel pump or ignition(spark) system. You may have a fuel pump or powerpacks that are starting to fail.
Just my 2 cents, I am not a mechanic put have experienced these problems on my sleds and outboards and these were the fix.
Also agree with Wally - could be the hose / primer bulb. Years ago I had a 150 HP Evinrude Intruder. It performed well at low RPM, but started choking at high RPM. As it turns the check valve in the primer bulb let loose and lodged itself on the leaving side of fuel line which restricted the diameter. At low RPMs it performed fine, but at high RPMs it was starved for fuel.
I'm a boating junkie and enjoy wrenching on my own sleds and outboards. Over the years I've had plenty of outboard issues and all but one was fuel related. Fuel delivery issues easily account for 80% of outboard issues.
And if you have spark, its time to move onto fuel delivery. Have you checked your fuel filter, cleaned your carb???? 9 times out of 10, older outboard problems are always fuel related.
What year is your motor? Fuel injected or carburetor?