Re: 66 Evinrude 40 hp problem
If when it kicks in there's a sudden boost of power, then you're looking at ignition difficulties. If it's sort of gradual, suspect fuel problems such as too rich, water in the fuel, dirty carb. If the outboard is 4-stroking like JB says, the spark plugs with be black and clogging with carbon deposites. They may provide a clue.<br /><br /><br />Did you try that spark test? A tank test will often not show the problem since the load is very different unless a test wheel is employed. <br /><br /><br />If you suspect ignition problems and the spark test proves the magneto is healthy, look at your kill switch wiring. If one of those lines are grounded, it will kill spark in the associated cylinder. Other possibilities are a faulty vaccum cutout switch and cracked spark plug wires.<br /><br />Spraying carb cleaner at the carburetor or mixing it in the fuel is an exercise in futility. Don't bother. If you have carburetor cleanliness issues, it must be disassembled, cleaned and rebuilt with new parts.
<br /><br />It's "running on one cylinder" and you are at a loss as to why it is down on power and sucking the gas can dry? <br /><br />What have you done (besides check for spark) to try to isolate the trouble? <br /><br />How do you know it is running on only one? Are you sure it isn't 4 cycling from over-rich mixture? That is what it sounds like to me.<br /><br />If you have a stuck or sunk float or the float level is set too high in the carb you will get too much fuel (uses fuel like crazy) and the engine will hit only on every other compression stroke (four cycling) which might sound like it is only running on one.<br /><br />I would wring out and overhaul the carb, by the book.<br /><br />Good luck. 
