"New" 15hp 2010 Mercury intermittently runs great, then the next second, rough.

Hi guys. I just purchased a 2010 Mercury (Tohatsu) 15hp 4stroke. It appeared to be rarely used as there were no scratches on the prop/foot. It ran great on muffs at the previous owner's house. Today, I took it out for the first time.  Initially, it cranked great and idle smoothly. I let it warm up for about five minutes, and proceeded to slowly open the engine up. It was accelerating smoothly until it got close to WOT when the RPMs dropped and the engine started running rough. After that, the engine would intermittently idle and run under part throttle fine, then the next second, go back to rough with reduced RPMs. It would try to accelerate, and sounded fine, until it got to half throttle or se. Then, it would start coughing and generally being unhappy with life - going back in forth from running great to running rough.I pulled each plug wire, and each time the engine died. I used a fuel tank I already had with fresh gas. Any ideas?

Check the entire fuel system for leaks.. also try to pump the bulb when it runs rough to rule out a bad fuel pump

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Have checked or replaced the fuel filter?

If you plan on doing your own maintenance, I recommend buying a shop manual for it. That will have a troubleshooting chart, plus good exploded views so you have a chance at repairing it yourself.

Water pump "peeing" ok? On muffs, the hose pressure is forcing water into the system and you don't get a good idea of the pump's condition. Then when actually on the water, where the pump has to pull water from the lake and force it into the engine, if the pump output is low, the engine can run warm. Some engines have a limiter built into the EFI and/or ignition system such that when the temp gets to the programmed threshold, it will cut ignition, effectively reducing the engine output. This is done to let you know that it's got a problem. Iv'e seen the sensors die and cause it too, but being that it's a motor that doesn't sound like it's been used much, I'd for sure check the impeller if you haven't already. Also look at the cup and plate while the pump is apart. When they sit a while, the impeller tends to develop a memory and one or more vanes may be "stuck bent".

Thanks for the replies.

I don't think there is a fuel leak in the system, After taking the cowling off, everything looked okay.

I'll change out the fuel filter. I didn't change it out when I was doing the initial maintenance.

That is a good point on the impeller. I did check to see if it was peeing, but didn't really take note of how much volume was coming out.

I believe the tank that came with that motor isn’t vented. If so, the cap needs to be loosened when running or it’ll starve for fuel.


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...sounds like a plugged main jet..drop the bowl and clean it..if ya got an air compressor that would make it easier.;

I had problems with my 30hp EFI Merc. It was the fuel line's inner lining disintegrated, intermittently blocking the fuel at the primer bulb and the connectors. I bought better fuel hose, cleaned the bulb and connectors, and all has been well since.

John

Thanks guys for all of the responses. I will start to investigate. Hopefully it is a fuel issue and not something major with the motor. I'm inclined to think it is since it runs fine when it isn't bogging down.

Guy's, wanted to give you an update. I put Sea Foam in the gas tank, took it out, and after a few minutes it started running great! The stuff must work!

Thanks for all the help.