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It sounds to me like a twin cylinder engine that is only running on once cylinder.
Start the engine and then, with a good set of insulated piers, pull one of the spark plug wires off and note how the engine runs.
If the engine dies, then you know that the other cylinder is not working.
If the engine runs the same, then the cylinder that you removed the plug wire from is not working.
If the engine stays running, but runs much worse than it did, then both cylinders are working.
What could cause one cylinder not to work?
1. Bad ignition coil
2. Valve rocker arm came loose. or bad valve.
3. Broken connecting rod on that side,.
4. BAd spark plug on that side.
5 Blown head gasket on that side.
If this is not the problem, contact me and we will look at this again.
Are we here?
I mean to say..
It sounds to me like a twin cylinder engine that is STILL only running on once cylinder.
I actually formed my answer before I read What you had already tried. So I was not surprised when I read about the plug.. But, it still sounds like it runs on one cylinder under load.
Let me know of your follow ups.. Thanks
I have located a complete Service manual for your engine.
Thanks for the bonus..
This is a little bonus for you.