1975 Johnson 4HP 4R75, 4W75 Outboards Service Manual, Page 13Get this manual
TROUBLE CHECK CHART (CONT) TROUBLE 2LOSS OF POWER(Assuming Ignition OK) (Cont)
POSSIBLE CAUSE BCARBURETORSee Section Poor fuel mixtoo much lubricant Carburetor adjustments(too leanbackfires) (too richexcessive fuel) Choke not operating properly Air leaks at packing nuts or core plugs Inlet needle and seat worn or sticky Boss gasket damaged Incorrect carburetor float setting Altitude horsepower loss CFUEL PUMP AND TANKSee Section Faulty fuel hose (poor clamps or seals) (kinked) Fuel tank or pump filter plugged Fuel filter restricted Fuel and vent valves not opening Valves not operating Diaphragm leaking or damaged Fuel system hoses plugged"0" ring damaged in connector DEXHAUST GAS ENTERING CARBURETORSee Section Exhaust cover screws leaking Cover plate gasket damaged Exhaust relief obstructed Lower motor cover to exhaust housing gasket leaking Cracked exhaust housing Exhaust housing to lower motor cover screws loose or missing EOVERHEATING POWER HEADSee Section Lower motor cover to power head gasket improperly installed or damaged Head gasket leaking (warped cylinder head) (water in cylinders) Water intakes obstructed Pump housing air bleed restricted Water passages or tube obstructed Pump plate not sealing (bottom) Pump impeller damaged Pump housing or plate worn Pump housing seal worn (driveshaft grooved) Water tube improperly installed FLOWER UNITSee Section Propeller hub slipping Bent or worn propeller Exhaust outlet damaged or clogged Bent gear housing or exhaust housing (broken driveshafts) Bent propeller (vibration) GEXHAUST GASES ENTERING COOLING SYSTEM See Section Pump plate not sealing (bottom) Damaged water tube grommet or "0" ring Pump housing seal damaged (5" adapter seals) Exhaust outlet gasket missing Exhaust housing gasket damaged