Do you run "Mercury Quickleen" or Seafoam in your fuel on a regular basis? It's important to do so, especially in:
1) Two stroke engines, and
2) Engines that idle a large percentage of their running time.
Two years ago I failed to use Seafoam or Quickleen regularly. The result was exactly what you describe: An increase of smoke and nasty fouled plugs. I'd clean the plugs and they'd foul again quickly. The smoke was bad, even though I use Mercury Premium Plus oil which is one of the least smoky available.
Research revealed that carbon deposits had built up in my engine. You can prevent them in the first place by running Quickleen or Seafoam regularly (I do so at least every other tankful now, and usually every tankful). But if you let it build up you must do what's known as a "decarbon" on your engine.
This involves running an extra-rich mixture of (usually) Seafoam through the engine. You set up a standalone outboard-style tank with some fresh fuel and several times the normal ratio of Seafoam. Hook this up to the engine in place of the normal tank, start the engine, and let the engine warm up normally. It will smoke like a chimney the whole time.
Once warm, run the engine at various RPM's for a few minutes, then shut down the engine and let it sit for a few minutes so the residual fuel/Seafoam mixture can loosen more deposits. Restart the engine and repeat the cycle. Do this for a while (we did it for at least an hour). Every time you start the engine the smoke will be unbelievable - a combination of the additives in the Seafoam and the carbon deposits getting burned off. Do this late some afternoon when everyone else is off the lake or they'll be either angry, or convinced your boat is on fire.
Take along the tools necessary to remove and clean your plugs. It's likely they'll get seriously fouled while all that crud gets loosened up in your cylinders and on your pistons.
When you're done, reconnect your normal fuel tank and run the engine until the smoking stops (to insure none of the ultra-rich mixture remains in the engine and fuel system). Remove, THOROUGHLY clean, and regap your plugs.
Your engine is now decarboned. Keep it that way by running either Quickleen (available from any authorized Mercury dealer that sells their oils, etc.) or Seafoam (WalMart, NAPA, auto parts stores, etc.) in at least every other tankful of fuel. In addition to keeping your engine decarboned, this also keeps your injectors clean so they can open and close properly, resulting in better mileage and smoother operation.
Do a Google search on "decarbon engine" for more info. Add the word "boat" if you want to narrow it down a bit.
Hope this helps!
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