2000 tohatsu 90hp with rough idle in gear or neutral. Smooths out on plane with plenty of power. Trimming way up lets it idle much smoother. At trim point where its better, exhaust starts coming out of prop hub, when rough, exhaust coming only from idle exhaust port. Motor is at correct manufacturer rec height on transom. Checked compression...all three good in limits. Set idle mix on all three carbs. Set idle speed.Good clean fuel. New plugs properly gapped.Also seems to foul plugs when trolling, some gray smoke in exhaust. This moter will run ok on the trailer at the dealer, so a mechanic will neeed to come to it at my dock. I need to get smart about possible causes before we start chasing ghosts this summer. Any advice appreciated.