Hi Norm ,BT and Mf,
I would disconnect the spark plug wire and pull the motor over and if there isn't any kickback with the spark
wire off then I would replace the spark plug as I've had a few that an old plug would advance fire.
As BT mentioned valve clearances must be checked ,also I have heard of a few problems with these motors when old like
worn timing gears that can move a tooth.
As you say Norm as it's not your mower it's not worth getting too involved but may be worth checking the quick easy things.
Cheers
Max.
