Re: 120 evinrude still acting up
Test the spark with the spark plugs removed. The spark should jump that 7/16" gap with a strong blue lightning like flame... a real SNAP! Does it?
Recheck the compression which should be approximately 100 psi and even on all cylinders. Is It?
Spark plugs should be Champion QL77JC4 plugs, and gapped at .040 (I know the book says .030). Are they?
If the area above are as they should be, and assuming that you haven't retarded the timing, and taking into consideration the fact that according to you, after having the timer base and powerpack replaced the engine ran fine for three (3) trips before reverting back to the same problem...... I would suspect a possibly intermitent short at ther ignition switch which would cause a slight amount of voltage to be applied to the black/yellow wire (kill circuit) of the powerpack.
Check that possibility by removing the black/yellow wire from the back of the powerpack. If the problem ceases to exist, replace the ignition switch. Note that it will be impossible to kill the engine via the ignition switch with that wire removed.
Note.... If the carburetors were at fault, replacing the timer base and powerpack previously would have had no effect on the problem.