Ignition Switch boxes 7.5hp

[Hughjohnson]

1977 7.5HP Mercury Outboard engine. Last year replaced switch box with new CDI 114-6222 switch box. It was tested in tank and winterized. This spring tank tested OK and then used on the boat for 2 separate days. On the third day the bottom cylinder stopped firing and testing showed no voltage from switch box to primary of the bottom cylinder coil.Everything else OK. Has anyone else had problems with switch boxes and is CDI a good make. Do you know if Sierra make a switch box for this engine. Serial No. 7097315. I am reluctant to buy another CDI unit and have the same problem. Hope someone can help.