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Tohatsu MFS5B Troubleshooting
The Tohatsu MFS5B is a 4-stroke 1-cylinder OHV (overhead valve) gasoline outboard engine with a rated power of 5 hp (3.7 kW) at 4500-5500 rpm. The engine cylinder bore is 2.32" (59 mm) and piston stroke is 1.77" (45 mm). A 7.5 cubic inches (123 cc) outboard motor has a carburetor fuel system, mechanical fuel pump, wet sump lubtication system, starting system with recoil starter, flywheel magneto capacitor discharge ignition system, six trim position, manual tilt, and handle tiller or remote control steering. The integral fuel tank capacity is 0.3 gal (1 liters). The propeller is made of aluminum and it has three blades.
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| Engine will not start or starts up hard |
Gas tank is empty - Fill it up.
Wrong grade of gasoline - Use quality fuel.
Dirt and debris in fuel system - Remove debris and dirt.
Spark plug wiring connection is defective - Inspect spark plug wiring connection, change or repair.
Faulty spark plug - Change spark plug.
Plugged fuel filter - Replace fuel filter.
Carburetor components worn out - Repair or replace parts.
Dirt in the fuel lines - Fuel lines must be clean.
Fuel tank air vent closed or restricted - Fuel tank air vent need to be opened or cleaned.
Fuel pump malfunction - Change or rebuild fuel pump.
Damaged or not adjusted choke cable - Adjust correctly or change the cable.
Improper valve clearance - Adjust valve clearance.
Wrong valve timing - Set the valve timing properly.
| Engine starts and immediately shuts off |
Damaged spark plug - Replace spark plug.
Faulty or clogged carburetor - Service or repair carburetor.
Ignition coil is faulty - Change ignition coil as required.
Fuel tank air vent restricted or closed - Fuel tank air vent need to be opened or cleaned.
Faulty fuel pump - Install new fuel pump or rebuild it.
Choke system is damaged - Change choke system components as required.
Excessive valve clearance - Adjustment procedure required.
Valve timing is improper - Set the valve timing properly.
Valve springs are weak or broken - Install new valve springs.
Valves sticking - Clean the valves.
| Rough engine running |
Defective ignition system - Test ignition and replace failed parts.
Dirty or faulty fuel pump - Check fuel pump and change if required.
Carburetor is dirty - Wash, clean and adjust.
Restricted fuel lines - Service fuel lines.
Fuel tank air vent clogged or closed - Open or clean air vent.
Fuel filter is dirty - Replace or clean fuel filter.
Gaskets are damaged - Replace gaskets.
Water intake is clogged - Need to clean or wash.
Choke system malfunction - Check and repair choke system.
| Engine overheated |
Engine overloaded - Reduce load.
Too lean fuel mixture in carburetor - Adjust carburetor properly.
Ignition timing is incorrect - Set the ignition timing properly.
Damaged thermostat - Change thermostat.
Faulty water pump - Change water pump as required.
Propeller selection improperly - Install correct propeller.
Damaged or bent propeller - Replace propeller.
Outboard engine is mounted too high or too low - Position the engine correctly.
Insufficient engine oil - Fill up the oil to required level.
Faulty oil pump - Oil pump need to be replaced.
Water intake is clogged - Wash or clean as required.
Wrong valve setting - Adjust valves correctly.
| Abnormal engine knocking or noise |
Stale fuel - Change fuel.
Engine overloaded - Reduce load.
Low idle speed adjustment is incorrect - Need to adjust idle speed.
Fuel pump is damaged or dirty - Clean or change fuel pump.
Main bearings or crankshaft failure - Need to install new components.
Piston rings or cylinder bore are worn - Change faulty components.
| Engine power is too low |
Dirty or improperly adjusted carburetor - Clean or properly adjust carburetor.
Clogged or defective fuel pump - Test fuel pump and replace if necessary.
Ignition system failure - Check ignition and replace faulty components.
Worn cylinder head gasket - Install new head gasket.
Powerhead parts are faulty - Replace defective parts as required.
Valves are stuck or burned - Clean or replace valves.
Propeller selection improperly - Install correct propeller.
Damaged or bent propeller - Have propeller repaired or replaced.
Motor mounted at incorrect height on transom - Check the outboard motor mounting height.
Trim angle is incorrect - Make the correct trim angle adjustment.
| Excessive fuel consumption |
Carburetor main jet is improperly adjusted - Adjust it properly.
Faulty or loose spark plug - Tighten or replace spark plug.
Worn piston, rings, or sleeve - Replace faulty components as required.
| Strong engine vibration |
Damaged or bent propeller - Replace propeller.
Damaged propeller shaft - Propeller shaft needs to be replaced.
Weeds in the propeller - Remove weeds from propeller.
Engine mounting bolt is not tightened - Tighten bolt.
Steering pivot is faulty or loose Tighten or replace.
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