time to take out the VOM and check things out... when did this start happening ?? recently or is this a new to you engine ???
normally the switch A post has B+ at key on position
start with this...as a guide anyway and you will need to learn diagrams and trbl shooting if you plan servicing a boat !!
https://maxrules.com/graphics/omc/wiring/1973-76_18-25.jpg
could have been bad gas ... you should fully inspect the rig before assuming it was pampered. carbs could be plugged up etc... tank pick-up plugged up , bad venting, bad hose
did you check/replace gear oil ??
"Good running" are the key words. I also look on how it was treated
from pristine to banged up. Can you do a run test or as is ..? tank or no tank ...around my way $ 500 cdn seems ok for a good running, clean 1980s grandpa motor if you are patient.
I am not buying much motors now but...
does not seems to affect ETEC injectors so much as valves. They do get dirty and require shop cleaning when performance heads down but a lot more problems comes from bad gaz gas-water mix dirty old fuel etc...
Seafoam is like vaccines... some believe in them some don't. The are several utubes reporting significant improvements after it's use but who posted such conclusions is never certain.
If it helps free up clogged rings maybe the improved compression helps...who knows.
As long as damages...
my 1966.... 9622a 9.5hp 1970s Merc 4hp and all others since the 60s & later when e10 existed have all worked fine with lead, no lead, ethanol across 50+years of boating...
I think the solution is regular use , store with FULLY fueled or fully "run dry" carbs. My 75hp ETEC...
2 strokes get 30-40% of their fuel oil mix sent out the window of open exhaust ports when both upstroke inlet ports meet downstrokes open outlet ports on pistons and the new load of fuel oil comes in the cyl.......... only to escape.
That is why DI engine are so economical ...