Mercury

It idles fine, runs great for a while then suddenly loses all forward momentum. i can put it at full throttle and it kind of lugs down. i am not a motor guru by any stretch. i k apart the lower unit and could not see anything wrong with it. i replaced the fuel filter and put in new plugs. any suggestions?

Yes check all the fuel system as the other have suggested. but i would say your stator is going bad. the stator is a coil located under the flywheel that supplies the power to the switch box that in turn fire the coils, now your saying but it idles fine. and runs a while before it starts loosing power. well the stator has 2 sets of windings a low speed set and a high speed set, when the high speed winding starts to short out the motor wont turn up past 1800-2200 rpms because this is the range the switch box changes from the low speed winding to the high speed. so why does the stator go bad? there is a thin copper wire that is wrapped around a steel laminated core. so as the wire and laminate heat up they expand and after man y cycles of expanding and contracting you guessed it, the insulating varnish wears off and the next time it gets hot you get a short until it cools down again. since this is an intermittent problem take a timing light with you and a friend and hook your light to the motor, have your friend watch and see if your loosing spark. note this was such a common problem on these motors mercury had to redesign their stator because they were having to replace so man y under warranty. they call it a red stator kit. i have personally replace hundreds of stators just on the 45 hps alone. hope this helps.