I have a 2001 115 hp mercury 4 stroke, model # 115elpt4s, serial# ot389155,6000 max rpm, it is on a 19 ft seamaster. I bought the boat and motor used in January 2005. When I first took the boat out it made 5000+ rpm but ran a little rough. So... I replaced the spark plugs, changed the oil and filter, inspected the fuel filter element, changed the fuel/water separator, changed the lower end oil. <br />I then took the boat out to check it out. It ran fine at first then started to run poorly. It started to loose rpm's and started to miss and run rough. <br />Three months later, three trips to the repair shop later, and a change in prop pitch from aluminum 19 pitch to ss 17 pitch, I have a motor that occasionally runs rough and will NOT make over 5000 rpm. usually 4700 to 4800 rpm. The repair shop does not know much about these new fangled 4 strokes but he says he has run all of the diagnostics and everything is up to spec. Any Ideas?
<br /><br />If the motor checks out okay, I'd start looking at the fuel supply. Specifically for restrictions in the fuel line like kinks or leaks. The pick-up in the tank could be clogged, the antisyphon valve may be sticky.<br /><br />Now for the weird one. If you have one of those cheesy crappy they-shouldn't-be-allowed-to-make-them aftermarket fuel lines, Junk it and get a fatcory fuel line. Especially the primer bulb. There isn't an aftermarket primer bulb I'd sell to my worst enemy. They're ALL crap! Johnnyrude, Merc, and Yamaha all have good primer bulbs. Attwood, Sierra, Tempo, etc. primer bulbs ARE ALL CRAP!!!<br /><br />Also, make sure the primer bulb hoses aren't clamped with those common screw-type metal clamps. Those must only be used when the thing you're clamping to is metal. They warp the nipples on plastic connectors and since the primer bulb check valves are there, they can really mess with the function of those check valves.