Check for spark; pull a plug wire, stick a spare spark plug back in the boot. Ground the metal shell of the spark plug against the motor framework. Crank the engine over, if you have a "hot" blue-ish spark you're good. BTW don't forget that most controls have a kill switch that operates off a lanyard, make sure that switch is "ON" or the motor won't have spark.
If you do have spark, spray some fresh pre-mix (50:1 fuel/oil mixture) into the carburetor throats and see if it'll start. If so, and it then stalls as soon as the fuel you sprayed in the carbs is spent, you've got a fuel delivery issue.
You'll have to see if fuel is actually getting to the carburetors, so you may need to disconnect a fuel line off the carbs and pump fuel with the primer bulb. If you're getting fuel up to the carbs, but they're not drawing fuel into the engine, the carbs will need to come off for disassembly, inspection/cleaning.
Before working on the fuel system be sure to disconnect the battery cables. You'll need to remove the starter to get at the carbs for removal, along with the fwd cowling support and any other interference in the way.
If you get to the point where you're taking the carbs apart, I'd recommend you replace the float needles if they are spring-loaded. I had a ton of troubles with those on my '84 50-hp. I replaced them with newer-style needles that don't have a spring, and they work much better.
http://www.marineengine.com/newparts/part_details.php?pnum=SIE18-7062
If you have the carbs with fuel pump on the side, this is the packing/diaphragm kit you'll need:
http://www.marineengine.com/newparts/part_details.php?pnum=MER1395-9024
HTH & let us know how things turn out.........ed
