It idles fine, runs great for a while then suddenly loses all forward momentum. I can put it at full throttle and it just kind of lugs down. I'm not a motor guru by any stretch. I took apart the lower unit and couldn't see anything wrong with it. I replaced the fuel filter and put in new plugs. Any suggestions?
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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 many 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 many under warranty. they call it a red stator kit. I've personally replace hundreds of stators just on the 45 hps alone. hope this helps.
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How old is the engine? The fuel pump may be going bad. You need basically three things to make an internal-combustion work: air, fuel, and spark. All three must be properly managed. You took care of the fuel filter and spark plugs, and it idles well. My guess is that your air-fuel ratio is going wacky when you open the throttle, so you need to check the fuel pump (fuel pressure), injectors, or anything else that can affect the mixture.
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check the mounting flanges on the carbs. they have a tendency to break. next time it lugs down, try pumping the primer bulb & see if it comes back...that'll be the fuel pump. asap you want to check fire on all 4 and compression. you need 100+ comp. on all 4 with less than 12 difference on any 2. also, is it running hot??
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first check your fuel system and tank make sure there is no debris in the tank or line. always use fuel stabilizer in your mix or straight gas and fresh fuel. only get what u need for a season or each time you go out on your boat. if that's not it then I would say you might have valve trouble, either weak valve springs, worn seats or bent valves.
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You should check your fuel primer bulb. If this is restricting the fuel flow it would cause the symptoms you described. Also an obstruction in the gas tank vent would cause a vaccum and starve the engine. If you can, connect another portable tank with fresh fuel and see how it runs.
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