1988 G2 Runs then slowly loses power and stops.

This is the most frustrating tease, and I can't figure it out. Battery fully charged. Gas full. Oil good. It powers up (never to full speed) but moving, then runs for a good 20 seconds before slowly losing power then coming to a stop.

If I wait 30 mins or more then try again, it will do the exact same thing.

The exhaust seems a bit heavier then normal when running.

Help, please!