Intermittent rough idle

[jonnyboyee]

Late 1980's Johnson 110 HP. Runs fine at idle but then gets rough (although doesn't stall) for 20-30 seconds and then runs fine. This cycle seems to repeat itself intermittently every few minutes or so. I have run the motor at cruising speed for 2-3 hours and idle speed for probably 8-10 hours total this season and havn't noticed any real improvement. I was sort of hoping it was bad gas but now think that there may be other fuel system issue. Any ideas? Thanks.