Not many pontoon boats in my area.----But pontoons are generally slower through the water and may not develop as much water pressure.----Hence the different spring.-----Your local Mercury dealer will likely help you on this.
There must be a --Chriscraft ----Owners group.-----Or Antique boat club online.-----Many of those folks are very helpful.---Boat museum in Clayton , New York is also a source.----Was boat properly winterized ?
I believe that site has the exact wiring diagram for your motor.----But do a complete evaluation on this motor BEFORE spending beer tokens on it.-----And --"great compression " -- means nothing to me.----Post the actual / accurate values.
The primer valve is fed by the fuel pump when the engine is cranking.----Or when the bulb on the hose is squeezed.------And fuel from the primer valve does NOT bypass the fuel pump.---It bypasses the carburetor bowls and metering circuits !----Lots of used factory ( and good ) primer valves are...
There must be a leak.-----Leaking float valves?--Ruptured fuel pump diaphragm?----Bad primer bulb ?---Air leak between bulb and fuel tank , including inside the tank.----All simple for you to check.