1986 20 hp Evinrude, electric start tiller control on a 14 ft Sylvan Super Snapper. Engine fires right up every time runs great all the way through the power bands. Boat will just hit 22 mph with my wife and myself in the boat. My question is, at idle the engine shakes from side to side. Once you begin to give it throttle, it seems to smooth out. I have installed new plugs and then fired it up and let it run for 6-8 minutes on idle, then shut it down and pulled plugs to check that both cylinders are truly firing at idle. Yes, both cylinders are firing at idle. Any ideas as to what may be causing this?