I know theorically of the decompression valve being there for the starting of the moped since it can help you getting some momentum thus making the pedaling a little bit easier. Also, in case you need to turn off the engine in case of an accident or before crashing. It will make the engine lose compression and thus turn off. However, physically to see an extra hole in there for me was kind of weird. But now, thinking about it, that hole makes the air to travel out of the engine all the way from down there, creating like a vacuum and turning off the engine. Unless you think different, let me now if I am wrong. Another thing is that I always look to fix all the easy, common things first. No compression, one thinks of rings, pistons, or cylinder, the top end of the engine. Other thing is weird for me is that people tend to be to exaggerated with respect to mopeds. I have 11 of them and none of them had a broken seal on the bottom end. These things were made to last. When in need of an advice people go for the most difficult part to replace (the seal). I dont know if they do it on purpose, for one to ruin everything or to look for professional help. Anyways, I'm placing a new cylinder jug, crankase made of thick gasket paper before I go down further into the engine. Forgot to mention, that the guy who sold the kinetic to me was a 350 pounder guy and I guess that was also another reason why I thought the piston was stressed out. Regards!!!