Post the frame serial number and we will check the year.
First check to be sure that the oil level is not too high. It should be approximately 1/4 inch below the full mark. High oil can foul the plugs.
Can you tell which cylinder fails? It you run the engine and pull the spark plug wires off, one at a time with an insulated pliers, each time should reduce the rpms of the engine. If you pull one that does not change the engine speed, then that is the cylinder giving you problems. You then have to figure out if it is compression, spark or fuel.
Check to be sure that the choke butterflies are operating properly on all of the carburetors. Make sure that they open all the way when you are not operating the choke. Keep in mind, that motor also has a manual choke on the front of the engine. If that is pulled out, the choke will always be on.
Before you get into the carburetors, be sure to check compression. It should be around 200 psi....testing with the throttle wide open.
Also....to start that motor, besides choking it, you need to advance the throttle almost all the way to give it plenty of air. Be ready to quickly pull the throttle back once the motor starts so that it does not over rev.
If you have not done much carburetor work....if you find that you need to clean the carbs, I highly recommend purchasing Honda's Marine Carburetor Manual (Available on Amazon) Here is a link to a partial manual that may help
http://ww2.justanswer.com/uploads/J...05k.marine_carburetion_manual_-_chapter_7.pdf
Mike