15hp 4stroke surge while idling

[Jon_123]

I have a 2015 15hp carb 4 stroke. The engine is barely used runs excellent, except while trolling around at idle. After trolling for a half hour or so the engine starts surging up and down. I cant seem to figure out the problem but when it surges up and picks up speed makes it hard to fish lol. Any thoughts? On why it might be picking up speed after extending use at idling speed?

 

[racerone]

My first " thought "----Is fuel tank properly vented ?

 

[Jon_123]

Ya I would say so. I just use a portable tank with the vent on the top. The engine seems to idle fine for about half hour while in gear...then it starts revving up for say 20 seconds come back down and continuously does that until you drive it hard for a while.