This is a DZ?
A condition I've seen with a stored engine is a stuck ring.
Excess fuel required to get it to fire and idles a bit, ring frees up a bit and goes rich and pops the prop.
Compression is higher and starter struggles. What are you using.
It does turn over by hand OK?
Personally I would bench mount to sort it - only knocking the airframe about while you get a handle on it.
The GP engines start up smoother with a PB switch in the GP lead - light the plug after getting the prop spinning,
avoids the occasional popping and banging/jerking on startup.
The prop must be kept tight, kept tight, kept tight, infinitum