Mercury 40 HP problem....HELP!!

[grubby]

Is it an inboard tank or a portable one?

[Black Crow]

I just had a pretty much identical problem with my 25 HP Merc. It would cut off on me at low speeds, would get bogged down after running a while, and finally got to where it would not run for more than a couple of minutes. Turned out that the motor was not cycling water and was not cooling properly and was causing the motor to seize up, which was an impeller problem. Get a repair manual for your motor ($20-$30) and it will guide you through how to get to the impeller. Check to see if it is worn or damaged at all. However, you really may want to consider saving up a little bit and have a mechanic take a look at the carb as well because they probably need a cleaning beyond what a spray carb cleaner can do. Also, if you take out the plugs to see if they generate a spark, which you should do, MAKE SURE you have the plug grounded against a metal part on the motor. If you don't it'll shock the daylights out of you (speaking from experience). Good luck.