Small Engines (Lawn Mowers, etc.): starting problems, engine model type, auto mechanic

Question
I have a yard machine with a 15.5 Tecumseh engine. Its always has started hard.It belongs to my father-in law and I'm a seasoned auto mechanic, so I know I have to repair this problem. It has a new battery, a couple of new starters,many of electrical checks.But still it bucks like the timing is off or it has a flat spot on the starter. It will start if I put a jump on it, but I know I can't keep jumping it every time It's need to mow. Any help will be appreciated.

Answer
Have you inspected the flywheel key?  It is rare to shear a key on these larger engines but it does happen.

Have you checked the valve clearance?

What is the engine model, type and spec number?  The engine model nubmer will be found on the engine, either stamped in the metal housing or located on a sticker.

The engine information will allow me to look up the valve clearances.

Eric