1978 Johnson 4HP outboards Service Manual, Page 16Get this manual
TROUBLE CHECK CHART (CONT) TROUBLE POSSI BLE CAUSE APOWERHEADSee Section Carburetor and magneto not synchronized Throttle control lever won't advance Air leak at manifold gasketswarped manifold (backfires) Broken leaf valves (backfires) Excessive carbon on pistons and cyl inder head Stuck piston, rings, or scored cyl inder CARBURET ORSee Section Poor fuel mixtoo much lubri ca nt 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 carbureto float setting AI titude horsepow er loss FUEL PUMP AND TANKSee Section Faulty fuel hose (poor clamps or seals) (kinked) Fuel tank or pump fi ter pi ugged Fuel filter restricted Vent screw not open Valves not operating Diaphragm eaking or damaged Fuel system hoses pi ugged"0" ring damaged in connector EXHAUST GAS ENTERING CARBURETORSee Section Exhaust cover screws leaking Cover pi ate gasket damaged Exhaust rei ief obstructed Lower motor cover to exhaust housing gasket leaking Cracked exhaust housing Exhaust housing to lower motor cover screws loose or missing OVERHEA TI NG POWERHEADSee Section Lower motor cover to powerhead gasket improperly installed or damaged Head gasket eaking (warped cyl inder head) (water in cyl nders) Water intakes obstructed Pump housing air bleed restricted Water passages or tube obstructed Pump impeller plate worn Pump impel Ier damaged Pump impeller housing cup worn Pump housing seal worn (driveshaft grooved) Water tube improperly installed Pump housing grommet leaks LOWER UNITSee Section Propeller hub slipping Bent or worn propel Ier Exhaust outlet damaged or clogged Bent gear housing or exhaust housing (broken driveshafts) Bent propeller (vibration)