I have a 1983 or 84 Johnson 60HP outboard on my boat.  This weekend while out fishing, my motor seemed to bog down, sputter and die, when I went from clutch in to about 3/4 throttle.  I could bring up the RPMs and speed to about 5mph but any throttle I gave it beyond that point would cause the motor to bog down.  If I caught the process early and hit the choke on the key as it was bogging down it would sputter, catch and get the boat on plane with no issues. As long as I stayed in the higher RPM range the motor worked fine.  When I slowed I could not bring the speed back up again with out it bogging down and hitting the choke.  I am assuming it is and air/fuel thing because of the choke making it come up but was wondering if those with more experience might have a suggestion.  I had this problem before and the mechanic said the fuel lines needed to be replaced because they were rigid and cracking instead of soft and flexible and fuel was leaking out.  I replaced all the fuel lines from the tank to the motor, internal lines from the hookup to the fuel pump, pump to carbs, and the return lines, things seemed to be fine early this season but now the problem has returned.  The lines are still soft and there was no noticeable fuel leaking under the cover.

Sounds like the carbs need cleaned.  The low speed jets are probably partially clogged.  

If they have a mixture adjustment on them, you might try opening the low speeds up another turn