What kind of a tester did you use? Did you have all the spark plugs out? Was the battery fully charged and capable of turning the engine at 300RPM with the plugs out?
The best testers have a solid ground to the block and you can set the gap and see all coils firing. The CDI unit that they sell at iboats is good, but you can rig them with as little as nails and a dry board.
My OMC manal refers to a neon tester model s-80 or m80, I can't find any info on this. Does anyone know where to find one or another method of testing?