The sheen. If it's new just start the engine and remove one spark plug wire at a time. If the engine changes with each wire removal you know that spark plug is firing. If there is no change when you remove one then that plug is not firing. A dead plug would also put a sheen on the water...the film.
If that all checks out then you might want to look at the mixture screw on the carb and back it off a 1/4 turn and see if it's any better. Sometimes corrosion around that port can cause issues as well...if so it can be sometimes difficult to tune it in.
All the best.
__________________
Mo
"Odyssey"
1976 C&C 30 MKI
The pessimist complains about the wind.
The optimist expects it to change.
The realist adjusts the sails.
...Sir William Arthur Ward.
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