if you turn the flywheel by hand and you hear a hissing sound, make sure you dont have an air leak.
seal the carb seat up with some liquid gasket stuff, and check like mike said the glow plug/plug spacer, and the exhaust while your at it.

if i try to turn the flywheel on an engine i got on the shelf i cant fully turn it over by hand, and its a used engine. and i dnt hear and air escaping