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Is this something that started suddenly ?-------------Check the oil in the lower unit.------Then check to see if there is spark on both cylinders.----Test compression.------Testing is required.---------But then it could be a bad sparkplug and minor carburetor adjustment as well.
When it seems to run fine on a flusher or in neutral but gags out when in gear/in the water (under load), that is usually the result of it "starving for fuel".
That is assuming that the motor was tuned up and otherwise ok.
You note a possible gas leak with the hose. That could be the cause. A dirty carb could be the cause.
Was the fuel "new" or stabilized?
Has it run ok (this season) or was 2 weeks ago the first time you have had it out after being layed up for a bit?
A little more history can help nail down "likely" causes.
eg. I got my motor out a couple weeks ago after it had been sitting in the garage all winter and this is what happened....