I've seen this when the valve seat started coming out of its seat. Check that first.

A loose seat was my first thought but the seat is fine. The adjusting nut has no resistance when tightning. In fact you can screw it on and off the stud by hand. The exhaust side nut has plenty of resistance. Both nuts look identical. Another thing i left out of the original post was, the rocker stud was also loose in the head. I was able to re-tighten it. Thanks

"A loose seat was my first thought but the seat is fine. The adjusting nut has no resistance when tightning. In fact you can screw it on and off the stud by hand. The exhaust side nut has plenty of resistance. Both nuts look identical. Another thing i left out of the original post was, the rocker stud was also loose in the head. I was able to re-tighten it. Thanks"

You do realize that there is a locking screw inside the adjusting nut? It should be torqued to 60 inch lbs. I can send you a Service Manual for that engine IF you like. Address below, put in proper format and remind me, model number and what you want.

I would have used some thread lock on that Rocker Arm Stud.

Walt Conner
wconner5 at frontier dot com

Update on the engine. Received the new parts, got them installed, adjusted the valves and so far the engine is running like new. Mowed for two hours yesterday. Used locktite on the studs. Want to thank Walt Conner for his help. Thanks

You're welcome and glad all is well now.

Walt Conner

Feasibility and estimated cost to repair/replace 18.0 ohc engine on five year old Murray riding mower?