Motor turns over but won't start

[zorro4040]

I had a problem with my Mercury motor on Saturday. Started and ran fine from the boat ramp - started fishing and after about 2 hours
was going to crank up and move. Motor turned over like the kill switch was off but never started. And the end of the day when I was going
to be towed in I tried it once more. Cranked right up and big cloud of blue smoke but ran fine back to the ramp. Next morning started and
ran like nothing was ever wrong with it. Serial no. OG429410 1996 EFI Any ideas?

[lpugh]

cannon plug, master relay, key switch, fuse connection for starters