4.3l Merc black/grey smoke--I need help with where to start troubleshooting

[Eric4479]

Today I took the boat out for a last run for the season. it's been several weeks since I've had it out but it ran great the last time out. Last night we had a cold snap so I put a heater under the boat cover (19ft bowrider). The outside temp was only 2 degrees below freezing for only 2 hrs so I thought the heater would be fine.

Anyway, I had it on the boat ramp and tried to start before unhooking the boat as usual. It was very difficult to crank so I gave it some extra fuel. It finally cranked over but ran rough with lots of smoke. I also noticed a sheen on the water that looked oily. When I returned to idle it stalled. After about 5 minutes of this I decided not to chance a trip out and came home to winterize the boat. The water that I drained out was very dark with an oily sheen on the surface.

What can I do to begin to troubleshoot this problem? Will I cause further damage to try to run it in the driveway with the muffs on? Thank you for your help and suggestions.

 

[GA_Boater]

If you have a carbed motor, I would start by looking at the choke. If the choke isn't opening it won't idle for beans and she will make black smoke.

I would run it on the muffs while looking for the cause.

 

[Eric4479]

Thank you. I will check on that tomorrow.

 

[Fun Times]

Have you checked the engine oil level to see if it's over full? Also check the oil for signs of water (milky looking oil). To check, either pull the oil filter to check what the oil looks like or blow some air down the dipstick tube which would mix the oil enough to show any possible signs of milky oil on the dipstick should it be present.

Also pull your spark plugs to see if they are fouled up any.

 

[Eric4479]

The plugs were fouled-rather back and oily looking. But the oil looked good in the dip stick so far. It needs changing but did not appear milky or look like milk chocolate. The oil level was at the full mark but not overfull. I did not try the air-down-the-tube trick so I can give that a try. or...just do an oil change and see what it looks like. All good stuff. thank you!

 

[hosteter]

Did you check the fuel?water in fuel sistem.

 

[Eric4479]

Thank you all for your responses. As it turns out, the oil was clean and, although it was not the choke, the problem was related to air intake. The spark arrester was SERIOUSLY dirty and clogged. And thanks to another post I was able to clean it very quickly with carb cleaner and forced air and finish up with brake cleaner so it dries quickly. This is my first year having the boat and it seems like it was not cleaned in years. Engine runs great now that it can breathe!