40 hp Evinrude 4 stroke Runs Rough at High Speed

[dreamen]

Took the pontoon out this weekend and it was running great, then all of a sudden it started jerking and like it was miss firing. When wide open it would pulse slow to fast over and over, put in neutral and it did the same. I thought it was running out of gas, changed tanks, started well and ran great at low speed. As soon as you increased the speed did the same thing. It is fresh gas, so that is not the issue. I understand this 4 stroke was made by Susiki? It has worked great till now. Any ideas?
Thanks

 

[Bosunsmate]

try manually priming the engine with the fuel bulb, it might be the fuel pump failing.
if that doesnt work check spark then compression

 

[dreamen]

Thanks for the reply, will try that. As long as it runs well at low speeds wouldn't the spark be ok? Or when you run at faster speeds the next cylinder kicks in? I had thought that maybe it was electrical like a coil was shorting out?

 

[Bosunsmate]

spark can be ok at low speed but fail at high speed, a spark tester checking that it jumps a 7/16 gap will show whether you have good enough spark that should work fine at all speeds.
When its playing up in neutral try a drop test, removing each sparkplug wire in turn with insulate pliers and seeing if you can isolate it down to a dead cylinder occuring.