Hi David 36pt,
Happy New Year to you as well.
I had trouble with my RCMK S520 when i tried to start it the recoil would lock up and then after a few more times it was stuffed i replaced it with another one then i found out about the Zenoah E-Z Pull Starter i fitted one of them and no more troubles.
I hope that might solve your problems.
kind regards,
Len.