I say you do not have spark !----So does spark jump a gap of 5/16" ( 8 mm ) on a test device , yes or no?----Have you pulled the flywheel to check the coils ?
Replace the prop and test run.----Or did you have lower unit off for water pump work?----Checked gear oil ?-----A 15" motor on a 20" transom?----Post as much information as you can,----Nobody can see or hear your motor!
Remove sparkplugs.----Put 6 drops of fuel in each hole.---Install plugs.----Try and start it.----Report what happens.---And there are no reed valves to worry about.
Fuel from the electric primer valve goes directly into the intake airstream.----This fuel bypasses the carburetor metering jets.----This strongly suggests that carburetors need attention