8 HP Evinrude losing power, missing

I have an 8 horse Evinrude that is giving me trouble. I looked up the serial number and it is either a late 80's or early 90's model. It started missing a couple months ago. I had forgotten to close the vent on the gas tank and it got rained on, so I just thought there was water in the tank. I drained the tank, fresh gas and oil, and tried again and it was a little better for a while but still misses. I will be going along on plane and all of a sudden the motor will drop out for just a second and then keep going. Sometimes it may only do it once or twice the whole trip and sometimes it does it over and over.

Today, it missed a time or two, but then it started losing power slowly until it was barely running. Seemed like a fuel problem like a blocked vent or air leak or something. I squeezed the bulb while it was going on and the bulb was not tight.

When I got home I checked the plugs, one is dirty but normal, and the other one is really black and oily. It actually had a drop of oil on the insulator. I held it against the block and cranked the motor and it had a good strong spark. I'm pretty good with engines but don't have much experience with outboards. Any help about what to check would be appreciated!

I also forgot to mention that I see a lot of oil and gas bubbling up in the water around the motor when it is sitting and idling. I assume it's unburned fuel from one cylinder not firing well?

Also, every time I get in the boat from the dock and it rocks the boat around, it makes the motor stall. The first time it happened I thought it was just a coincidence, but it stalls every single time I get in the boat.

First i would pull the bowl off the carb and see if you have water or trash in it. then take your finger with real light pressure lift the float until the needel is seated. release and make sure needle isnt sticking. if that is ok put bowl back on. then put in new plugs and get a can of seafoam, mix a heavy dose in tank and run motor pretty hard for a while next time you go to lake. I would recomend if it is possible to tie boat off front and rear at dock, just put it in gear and run about 1/2 throttle for about 15 minutes or so. Then take her out and give her a good wide open run. Probably a little water and some gum in carb. Unburned fuel is from that one plug. You might also check and see if some one has a spray cleaner called Deep Creape to spray inside carb while bowl is off. Seafoam in spray can.

Thanks for the advice! I put a can of Sea Foam in it when it first started missing, and it ran better than it ever has until that tank of gas was gone. I will do that again and replace the plugs, too.

That black stuff in the water is in fact fouling from a cylinder that isn't firing correctly.

You say it is a late 80's or early 90's? Back in those days, the fuel lines they used inside the motor was not made to handle ethynol fuels of todays world. Tons of boat dealerships, chainsaw places and small engine shops in general have had issues relating to the fuel eating away at those fuel lines. I'm not sure when the last time you changed yours out but that is where I would start. Seems you are losing pressure someplace and sucking air. I'm speaking of the fuel lines on the carbs. I would also perform a carb cleaning or rebuild if necessary (probably due). Might also be a good idea to change out the plug wires and possibly check the mag to see if the clearance is correct and ensure it isn't corroded.

Lastly, I'm not sure if you "fog" your engine or not when you put it up but I've always advised against it. Reason being is that it usually takes several trips, tanks of fuel and a couple of plugs to get it running right again. Good luck. I'll bet you will find it is something as simple as a fuel line!