Rough Running 2004 four stroke 15HP

[Fisherman Tom]

Howdy folks,
I’ve got an outboard that idles roughly and is quite smoky. It starts right up, trolls at low rpm for hours, and runs strong at WOT, but it seems to be smoking. It’s got low hours, has been professionally maintained, new plugs, and a freshly rebuilt carb. I haven’t replaced the diaphragm on the carb, but it looks good. I also haven’t replaced the intake gaskets, but they look good too.
I was burning fresh, high-quality gas through a new fuel line today, but when I put it in neutral today, after trolling for a few hours at a low rpm, it would idle at a normal rpm and then suddenly jump to a higher rpm without me touching the throttle.
Any Thoughts?

 

[racerone]

Checked oil level ?----Sudden changes in RPM point to an ignition issue.-----I would do a spark , maximum output test.----Or perhaps as simple as a bad sparkplug / wire / sparkplug boot.

 

[Tony Outboard]

If you are doing a lot of trolling I would take a look at the plug too.