Hello Chuck,
Sounds to me like a classic air leak problem. I would try resealing the backplate with some silicone sealant. The other possibility is the O ring sealing the venturi could have become hard and is leaking. Backplate would be favourite though!
The LA piston and liners don't last as well as some other makes. I have had two wear out sofar. My problems was poor compression and difficult starting. When the liners go bad, you can usually see the brass showing through the very thin nickel plating. The P/L are cheap enough to replace if that is the problem. My limited experience is that the effect is gradual. If you have had a sudden change in the way the engine runs, then almost certainly an air leak somewhere.

Andrew.

Yes. The liners go. Coloration variation on different sides of the the piston on an old engine could be normal blow bye from normal wear. The crank pushes one side of the piston up against the liner, harder than the other side. The forces, side to side, are asymmetrical. It's also possible that the liner has become slightly oval, after being run for years. From what I understand the liner is thinner than the FP liners that preceded the LAs. I've seen FP liners wear through as well. Tell tail scrapes on the nickel, when the brass shines through that P/L is cooked. Right. Back plate, o ring on venturi, look into that. Bushing to crank leakage can also be problematic in worn engines. I've even heard of a needle valve that wore out (vibration?) clearance on the thread to needle opened up, didn't seal. When a new needle was put into the needle valve, run returned to normal. Might be the airframe as well. Loosening up of structure, softening of nose, can change the vibration harmonics of a given engine/airframe unit. Vibes on the ground differ from vibes in the air. Pin hole leak in the tank, not apparent unless the tank is pressure leak checked under water. Even a small barely detectable leak can cause havoc. Especially so if you are running muffler pressure.

OK Guys Thanks for all the tips. I think I will retire this one and get the new on out of the box.