Mercruiser 3.0 1987 engine not running well after warmed up

[alpha-newbe]

Engine idles fine and will run smoothly at lower rpm in neutral and in gear. After running smoothly on the lake for 15 min or so at 3500 rpm then running for 15 min at 1000 rpm smoothly all is good. Then i go to running at 3500 rpm again and it runs rough so i back the rpm's down to 1500 again and it runs smooth. So then i just take if back off the lake because it is not running well.

 

[alldodge]

Might be a fuel pressure issue (pump or fuel lines) but when was the last time the carb was cleaned?

Have you checked the points and the dwell