Weak spark. Have you cleaned the points and checked for 1 ohm or less resistance across the points. Do you have the Outboarder Club Magazine it has a story on troubleshooting ignition systems. A good spark will jump a 3/16 " gap or more in free air, a weak spark will jump .030 " gap in free air but won't fire under pressure.
Mercury -MK20, MK10, KH7, KG7,KG4, KE4,KE4, KE7, KF5,KF3,KE3,KD4
Wizard WH7, WD4
Johnson - QD15, HD26, TD20, KA38
Sea King 5, K1 (merc)
Evinrude- Foldlite 1930, ELTO - LW #309, 1951- 7.5, Zephyr, 68 3 hp
Bendix SMD , Champion 4.2 , Scott 7.5,