Sport jet is a FORCE powerhead.----Powerhead is no different than the outboard motor.-----Exhaust manifold is different.----Fuel systems must comply with Coast Guard rules for inboards.
Really-----Why do you say it is EXHAUST pulses that move the diaphragm on old outboards???-----Crankcase PRESSURE pulses move the fuel pump diaphragm !------Sounds like the engine has classic lean sneeze.-----And some of these units rev high for proper cooling at idle.
I have learned ( long ago ) that trouble shooting on outboards starts with a compression test.-----Post the numbers here.-----Walked away with a low hrs 1964 model 5.5 Johnson from an---- AOMCI----meet.-----Half a dozen folks looking at it basically declared it SCRAP because of low compression...
I doubt you will find any mixture adjustment screws on those carburetors--There are no mixture adjusting screws !!------Checked the flywheel key?----All wires from CDI box going to the correct coil ?------Checked the reed valves ?