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blue is 72 and up area
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from early 70s upward Merc was evolving to electronic ignition ex: with stator CDI ignition under control of a trigger coil ( no mechanical points) .
around mid 70 they had phase maker ignition a bit of a mix of CDI and points MAKING contacts to fire the points. Normal magnetos usually OPEN their contact surfaces to generate sparks but PHASE MAKER closes the points to fire.
here is a good read on Merc ignition by M.Mohat
https://wrcoutboards.org/wp-content/uploads/2021/02/The-Phase-Maker-CDI.pdf
https://wrcoutboards.org/wp-content/uploads/2021/02/Generator-Coils-Explained.pdf
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these are the ones I installed on my 7.5hp …much less expensive
NGK BU8H Standard Spark Plug, One Size, Spark Plugs – Amazon Canada
fyi
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Sometimes the serial number is on the TOP of the block….over by the Exhaust Port….back almost under the flywheel. Very faint Look there also.
I have attached some cowling pics. Once you determine what the year most likely is….you can determine the serial number range from the chart. Mercury parts are sold by serial number; not year.
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It’s best to never use the stop button to shut that vintage of Mercury down. Choke it instead. Also; never rope it over with the spark plug wires disconnected.
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new impeller , carb job and GC oil with some masking tape and Mercury black and it will look brand new ! dirty phase maker points are not so worrysome it semms …. see article
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They are great motors if they have been maintained. Check for leaks/water intrusion in the lower unit. Hopefully you have spark. Ignition components on those are expensive and can exceed the value of the motor.
It’s always best to work from a serial number. It’s probably on the block somewhere; you just haven’t found it yet.
The pic attached shows another location for the serial number.
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