Stupid ethonal fuel

If your old motor boat motor don't run like it used to, you may need a jet change!
Had a couple of old 70hp motors that ran great last winter but now run like they have a tater in the tail pipe. After a couple trips to the lake to test ignition at speed, we went smaller on the main jet. That made them scream again.
Lost some sleep and money on this deal. But it is so hot in houston right now, that
the birds are taking off their feathers. I guess the motors needed a altitude adjustment.

I'll bet that if you check the main jets using number drills, you will find that they are quite a bit oversize. As hard as it is to imagine, gasoline will erode the brass and enlarge the hole, making the motor run much richer and be down on power.

Most often this is seen in the larger motors with high time engines.