I did a compression test today on my 2000 Yamaha 50hp two stroke and the top and bottom cylinders read 120 but the center reads 80. The engine runs great and idles great. This engine has the best idle I have ever experienced as its low and quiet. No vibration or odd sounds. I can get up on plane fast and engine pushes the boat fine from top end speed to low end speeds. I just changed the plugs and ran the engine on the muffs for 20 minutes. The plugs on the top and bottom were clean but the middle plug was fouled and black looking already. I don’t know much about the engines history but I just completed taking the lower unit completely apart, replaced all rings on the gear case, changed shaft seals and everything looked good. Do I have a ring problem on the middle cylinder due to corrosion or carbon? I am going to do a ring free shock treatment this weekend to see if it helps before pulling the head. Then I will pull the head, clean the cylinders and change all the rings and gaskets. If I dont see any red flag damage what else should I do?