1974 Johnson 40 HP Outboard Motors Service manual, Page 14Get this manual
TROUBLE CHECK CHART (CONT) TROUBLE 2LOSS OF POWER(Assuming
POSSmLE CAUSE
APOWER HEADSee Section 5
Carburetor and magneto not synchronized Throttle control lever bent (won't advance) Air leak at manifold gasketswarped manifold (backfires) Broken leaf valves (backfires) Worn lower main bearing seal Excessive carbon on pistons and cylinder head Stuck piston rings, or scored cylinder or piston Recirculating system plugged CARBURETORSee Section Poor fuel mixtoo much lubricant Carburetor adjustment(too leanbackfires) (too richexcessive fuel) Linkage screws loose Choke not operating Air leak at packing nut Inlet needle and seat worn or stiCky Incorrect carburetor float setting Boss gasket leaking Dirt in high speed orifice FUEL PUMP AND TANKSee Section Faulty fuel hose (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 fuel line connector EXHAUST GAS ENTERING CARBURETORSee Section Exhaust cover screws leaking Cover plate gasket damaged Damaged exhaust hOUSing seal Adapter gaskets leaking racked exhaust housing Exhaust tube to cylinder screws loose or missing OVERHEATING POWER HEADSee Section Exhaust cover gasket leaking Inner exhaust cover leaking Power head gaSket improperly installed or damaged Head gasket leaking (warped head) (water in cylinders)