engine takes off or bogs while trolling

[tiger king]

I have a 1990 Johnson 40hp (model J40EESR) with the VRO disconnected. While trolling at 7 knots the boat will occasionally take off(to 12-15 knots) even though the throttle hasn't moved. It also less frequently will slow down. I then have to increase or decrease the throttle to get it back to trolling speed. It seems to sometimes be induced into taking off when a swell pushes from behind. I would really like to troll at 8 knots but the engine will not hold at this speed. Again, it just wants to take off. <br /><br />The carbs were recently blown out with carb cleaner and air(my friend) but no gaskets or parts were replaced. The fuel filter was replaced and a new fuel line/ bulb installed.