Mercury 50elhpt 2016

by tim smith
(Keithsburg IL)

engine starts and runs fine for about 1/2 mile and then surges up and down and almost dies. If I let off throttle it will run fine, If I go back to half throttle or above it starts surging again until I let Off, Engine never dies or fails to start. Just at half throttle or above and after warmed up. All maintenance done . New filters, cleaned and checked mechanical lift pump, installed fuel pressure gauge and has 47 p.s.i. right on the money all the time. I also changed tanks, primer bulbs and fuel lines to no avail.I bought it new and use it frequently. It never sits longer than 3 months when water here is froze up. I also removed VST and disassembled to check and it was perfect. No dirt or corrosion and fuel lines and float with needle and seat looked perfect. Im an automotive mechanic and have lots of test equipment, but Im not sure what else to check. I even thought that maybe TPS sensor might be flaking out, Im working on a jumper harness to check it with a DVOM while running it. Also it runs fine on the muffs and I cant get it to act up if I leave it strapped down to the trailer and back it in and run it hard. I think it may be a R.P.M. and load problem because when tied down I cant get the RPMs up where they are just running the boat up on plane.And the muffs put no load on the engine. Im lost and duck season is coming and local dealer said ive already done everything he would do. Any help would be greatly appreciated.