For Mercury parts & schematics, go here.
But given the model year of 1980, fuel lines back then were not alcohol rated. As well as a carb rebuild, I'd replace the fuel line from the fuel pump to the inlet manifold.
The thing is, carbs orifices are tiny to miniscule and to be properly cleaned and flushed they need to be cleaned and flushed in a specific direction and order, i.e., one flushes a miniscule orifice to a tiny one and then to the largest one where the crap can exit. Without a factory/OEM manual, it's a crap shoot and I've never seen correct or sufficient details in any Seloc or Clymer 3rd party 'service' manuals. IMHO they're worthless.
I'd replace the fuel lines, purge the system of all fuel and would then get Seafoam from NAPA ($6-8 liquid) and force it into the carb by getting it in through the line from the fuel pump. Do NOT attempt to start it, so remove coil wires from plus or remove any ignition kill. Get the carb FLOODED with Seafoam and leave over night. Drain the next day and fill up with fresh fuel and NEW plugs. Worn or wrong-gapped plugs also cause many starting issues.
Hope this helps ... otherwise Chris and Dave @ Perley's Marina Repair in Rowley are amongst the best Mercury guys around.