2007 Mercury 60HP EFI 4 stroke
Let me start by saying this boat sat for 3 years (2010-2013) and we had major varnish build up despite having Stabil in the tank. Before we ran it, we drained the fuel tank out and ran quite a bit of Star-Brite and SeaFoam to clean it out over the past 3 years. We have run lots of fuel line and injector cleaner thorough it as well over the years. The motor starts and idles well. Usually it runs perfectly at full speed and full throttle range for 15 mins to 1 hr. But then, will start to run rough and shakes like a cylinder is missing or its starving for fuel. When this occurs, it probably will only run at 2500 rpm (a guess as I don't have a tach). We have replaced all fuel lines from fuel tank to VST, both fuel filters, and the low pressure fuel pump. It still didn't run correctly. We then took VST off and cleaned it all out including needle valve, screens on high pressure pump and the fuel pressure regulator. We also replaced spark plugs properly gapped to 1 mm. We cleaned out the screen in the gas tank cap in case it was clogged and causing a vacuum. We still need to make sure it isn't picking up something in fuel tank that would restrict fuel flow under full load and stopping gas flow; we will do that with a new 6 gallon tank and new gas later this week.
Was thinking maybe a coil gets up to temperature and then starts to fail? Also read that neutral rev limiter switch can go bad and cause something like this, but figured it would happen all the time if this was the issue. Also, if injectors were bad or clogged, it wouldn't allow us to run at full speed for 2-5 mins wide open. Any help on whats next would be greatly appreciated. We did not have the tool to check high pressure fuel line, but thought that if it ran for 10+ mins wide open, it probably wasn't the high pressure fuel pump.
Thanks for your help, experience and advice.