Have Boat Questions? Ask a Boat Repair Expert.
Is the engine running OK now or what?
Is the engine turning over fast?
Put a voltmeter on the battery and see what the reading is, then while cranking. It's possible a weak battery is not putting out enough voltage to start the engine. Your engine uses battery voltage. Try this and post back the figures.
What did the spark plugs look like? Black?
Could be the engine isn't turning over fast enough. With one plug out there is no compression and the engine cranks faster bringing in the proper air/fuel and spark.
Charge the battery as well.
It's true long idle times can help cause fouled plugs in any engine 2 or 4 stroke. A engine running too cold can do the same. What I would recommend if you are idling a lot is to run a carbon cleaner such as Yamaha's Ring Free or Mercury's Quickleen in the fuel. These products will help stop or reduce fouling.
Make sure your thermostat is in place and operating properly, use a good quality, fresh fuel, of a Top Tier manufacturer. Use a marine 4 stroke outboard oil. All will help.