Intermittent spark is tough to diagnose, but follow these steps:
1. Make sure all electrical connections are clean and tight. This means taking the connections apart and cleaning the contact surfaces with emery cloth.
2. Remove the flywheel. Check to make sure the flywheel magnets are tight in the flywheel. Clean the magnet surfaces and the stator frame with some medium scotchbrite. Clean the breaker points with some fine grit emery cloth and contact cleaner. Check and reset the breaker points- the gap should be .018".
3. you say that the outboard wants to fire when you release the key, but not when the key is in "start" position? To eliminate the possibility of a fault in the key switch or wiring harness, disconnect the wiring harness from the outboard and try starting the outboard by jumping the solenoid.
4. If all of the above fails to produce consistent spark, you will need to perform a complete ignition diagnosis- you will need an ohmmeter, and can find the procedure at this site:
Carburetors- These are simple carbs, with a fixed high speed jet and an adjustable low speed jet. They probably should be removed, disassembled, and completely cleaned with carb cleaner and compressed air. Do the carbs one at a time, to avoid mixing parts. To set the low speed jets, the initial setting is 1 3/4 turns out on each jet. Then turn each jet in (clockwise) 1/8 turn at a time, until the engine begins to falter. Then turn each jet out 1/4 turn. Ideally, you would use a carburetor vacuum synchronizer to fine tune the carbs, but if you don't happen to have one, the above method will do the job.