auto cam chain tensioner
I got one for my '74 850 from: RMA Engineering, California,phone# 408 - 578 - 4032. Cost $110.00, it took a little work to fit it but well worth the effort, the instruction sheet is very comprehensive.You will find,if you fit that one, that the cam chain is tighter than with the stock tensioner, so far that does not seemto be a problem -- 600 miles since fitting it.
Good luck. James.