When my plastic cam gear went, the engine jut ran really bad (rough, low on power, etc.). More recently, when I had low compression in one cylinder and oil coming out of the breather tube into the carb, it was worn out rings on that cylinder. If you put a little oil in the cyl with low compression and test the compression and it goes up with the oil added, that points to bad rings. If it stays the same it is either valves or possibly a broken piston. In any case, this engine is very easy to tear down and overhaul. Parts are a bit pricey, but you can refresh the engine and have it running like new pretty easily and WAY cheaper than a new one (usually). I was a little intimidated to tear into a liquid cooled, twin cylinder at first, but the JD engine manual is very helpful and the engine is actually pretty simple.