Knocking/shutter at high RPM issue - Johnson 50

I am helping my brother with his boat because the powers that be forbade pouring any more money into it. The motor is a Johnson 50. In gear and in neutral it sputters and shakes when it’s pushed to high RPMs. Yesterday I cleaned the plug wires and it got up to plane but cut out after a few minutes and will only chug along. I’ve since put in new plugs and wires with no improvement. The carbs were supposedly cleaned by the mechanic a few months ago and they are sparkly clean inside when I remove the air intake plastic to look. I did spray some cleaner in and run it to no effect. He has used ethanol gas in the recent past because his mechanic told him it was fine (which I don’t understand). Here’s a YouTube video of the issue. Any initial reactions? https://youtu.be/h_X59Lw6FtM

He showed the video to a friend who thinks we should take the flywheel off. I still think it must be fuel related. Has anybody seen this kind of knock once you rev it up?

The venturi of the carbs can look just as pristine as possible, but if you have junk in the high speed fuel circuit it will cause problems. That's the long way of saying just because they were 'cleaned by a mechanic' it doesn't mean there isn't something clogging one or both of the carbs in the fuel circuits.

In the video it looks like the rod for the timing advance is moving with the throttle linkage, but you need to look under the flywheel and make sure the timer base is actually moving. If the timer base is sticking you won't have enough advance for the higher RPM and that could explain the mis-fire.

Also, did you confirm that you have good spark that will jump a 7/16" gap? If not, do that as well.

Don't do that as it will break piston skirts!!! Rev it up in gear only. Disconnect the brown from pack as it acts like its in overheat.

See the posting on June 10, 2018 by "spagency". He posts a video of the same condition on his 25hp Johnson.

I had the same thing on my 25HP. The impeller was putting water out the telltale but it was not enough to prevent overheating. My engine has the SLOW feature (Speed Limiting Overheat Warning), and will not allow full revs until the engine has cooled off.

That was my problem and it sure looks a lot like yours. Check to see if your engine has the SLOW feature. A new water impeller solved my problem.

Thank you! We are going to tinker with it again this evening and try out these suggestions and will post an update