Suzuki / Johnson 115DF 2002 no longer starts after backfiring

[fredinireland]

My boat has been sitting unused for a long time. When I tried to start it she fired up on all 4 cylinders but then started to stall as each cylinder shut down until it stalled. It would fire up and then misfire and stall even at higher revs. Seemed like lack of fuel. Also when trying to start it would backfire/pop. I removed all the fuel system and cleaned out. No bad stuff found. Checked HP pump was working and replaced everything. No start at all.
Any ideas what might be the problem? Nothing else has been messed with.

[fredinireland]

SOLVED: I ended up replacing all 4 spark plugs even though the old ones had hardly been used. Fired up with engine warning light but this cleared itself. Now seems to run OK. Who could have guessed!