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

[cricketchaser]

"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?

 

[cricketchaser]

Anyone?

 

[Texasmark]

Fresh fuel was going to be my answer so my second is to drain the bowls on the carbs. Ensure that when you have a bowl dropped, you can squeeze the bulb and force gas out of the carb but lifting the float to horizontal position, carefully with your finger, will stop the fuel flow. Bowls should be spotless. If crud is present clean your carbs with a good carb cleaner and lots of compressed air....wire in any small hole/tube you find. What you had was crud dislodged by your operating the engine. Question is, where is it?