Johnson 4 stroke problem

[homedad]

I have this 2003 Johnson 70 hp 4 stroke that starts fine and runs fine until 3400 RPMs. At 3400 it will begin shuddering badly as if it's no longer firing on all cylinders. I back it off to 3300 RPMs and it smooths right out. I've had it in the shop to find out whats up and the last guy said it was fine and obviously did nothing. When I got it everything ran fine during the test ride and 15 minutes after the guy left I got a CEL that indicated overheating. I replaced the impeller and have a good pee stream yet last night my son said an alarm went off and the overheating light came on. I don't think these 2 issues are related because the power problem is there even when it's cold (at start up). Any ideas would be greatly appreciated.

[FogHorn]

Broken or weak valve spring? That’s exactly how my old Evinrude behaved when that happened. It would run fine until high RPM then roughened up and acted like it was running on one cylinder.
One other possiblity depends upon how timing advances with your motor. If it’s a mechanical advance tied directly to throttle-plate position, then timing might be far off.

[homedad]

Thanks. I'm getting to the point that I'll check every idea anyone comes up with. Fixing this thing is becoming a new hobby.

[Bamby]

This could very easily be a vent or fuel delivery issue.. Portable tank? Are you opening vent or plugged up fuel filter restricting fuel flow or cracked fuel line allowing air to be sucked into the carb under higher load conditions.

[Liquid Asset]

I agree with the possible fuel vent issue. As far as the temp warning, has the thermostat been changed? A good pee stream doesn't matter if the thermostat isn't opening as it should.

[jrolin1]

In addition to the thermostat I would replace the poppet valves assuming a 4 stroke johnson has them like a yamaha or mercury.