I ran the motor today at the lake.
With the motor in the lowest trim setting (as if to put a boat on plane), it would barely run. Just sputtered and popped. Would not stay running long enough to get it in gear, and if I did, it did not have enough power to push the boat.
Put the motor on its highest trim setting and it purred like a kitten. Ran and started great that way, so we just ran around the lake and fished with the nose up. This was a no wake lake, so it really did not matter. In either setting, the lower unit was very low in the water due to the low transom on this boat.
I put the motor in a can this afternoon and shot a quick vid. I tilted the can to be able to get a similar angle to the transom. It behaved just like on the boat. Check the vid out. Breathing its own exhaust has no effect.
I do have a carb rebuild kit on the way. Only things I can think of now are a sinking float or something to do with the low speed needle and its seals. Both will be replaced soon. https://www.youtube.com/watch?v=GVqiH6oJ4II