I've owned the Scotts lawn tractor with a Kohler 16 hp since new. Last week while mowing at full speed the engine suddenly stopped while making a sound very much like a valve had let go.

I pulled the rocker cover off and was pleasantly surprised when I found that the 6mm bolt that holds the rocker arm to the pivot point had snapped and the rocker arm was detached. I ordered the bolt from McMaster Carr (bag of 50 for 10 bucks +shipping) and when they arrived I reinstalled the rocker and cranked the engine to make sure the valves opened on closed properly... which they did. I reassembled everything and went to start the engine but no joy.

Cranks fine but will not fire. Sprayed starting fluid directly into the carburetor with no change... no combustion at all.

Pulled the plug and checked for spark... plenty of spark but I couldn't vouch for spark when the plug is installed and under compression.

Took a compression test and got 60 psi... seems low to me but the compression release mechanism at cranking could be causing that.

So there you have it... from running fine to will not fire after the bolt that holds one of the rocker arms broke. God knows what other damage may have resulted from a valve staying closed with the engine at just about full throttle but clearly something has.

The 60 psi has me a little concerned and if it were one of the hundreds of car engines I've worked on (currently rebuilding an '83 Mercedes 380sl) I would have already pulled the head but I wouldn't want to pull the head just on speculation over 60 psi. when I don't know what the compression should be.

bTw... the engine has hydraulic lifters, an oil filter, overhead valves and must have an oil pump to get oil through the hollow push-rods to the rockers.

Any suggestions would be greatly appreciated.