did a compression test because it seems that he was running on only 3 cylinder.
and the result was 160 - 149 - 35 - 150psi , the 3 cylinder is the problem
what is it valve, piston

 

could be a number things, its a case of head off!

 

take the head off and get it checked out, if all is ok with it and the piston looks ok from the top then its more than likely that the side of the piston has cracked, this is what happend to mine number 3 had 30 psi all the rest had 160psi

 

mine had tottally melted down the side of piston 2 and 3

 

probably detted and cracked the ring lands just like pic above
been there.............twice lol