I was working on a JD LA115 with 31 Cu in Briggs that had sat at least two years. Long enough there was multiple problems that had to solve before final repairs.

The engine started on the fuel in the tank so I kinda figure it just a fresh dump by the owner attempting to start it before they sent the mower in. Apparently not the case. The engine ran fine off load other smoking due oil in the cylinder and muffler. The problem showed up under the deck load. When you first engage the engine struggled and knock heavily until back up to full speed. Considering this was Intek I pulled the rocker cover and did a leak down test checking a blown head gasket which it passed other finding a crappy installed plug wire terminal. So adjusted the valves and resealed the rocker cover. For some reason it dawn on me to drain the fuel and replace with known good fuel. Maybe it was that mower had sat two years and in the back of mind the fuel could be a problem.

After the RTV had set up I crank up the engine with the new fuel and the struggling and knocking was cured. Apparently I had a case of pre-ignition with old fuel. To be honest it is the first time I have ran across this particular problem. Oh I had lot of bad fuel cases in the 15 yrs I have been doing this work professionally but none like this one.

Has anyone else ever ran into this particular problem? Usually here when the is bad it is bad all the way.