Engine Trouble Shooting

These diagnostics were taken directly from the Cub owners manual. The complete owners manual can be found in the community forums under the link for PDF Manuals; NOTE: Manual downloads are available for community members only; don't worry, everyone is welcome to join our community.

Engine Will Not Turn Over
Cranking motor inoperative or defective Replace
Battery faulty Replace battery
Cables and terminals faulty Inspect ground cable and battery-to-starting switch cable for any faults which may cause shorting; also inspect for incorrect connections. Replace cables if necessary
Starting switch defective Replace starting switch
Internal seizure Hand crank engine with spark plugs removed and clutch disengaged. If engine does not turn easily, internal damage is indicated.
Engine Turns But Will Not Start
Fuel system faulty
(a) No fuel in tank
(b) Fuel strainer bowl screen clogged
(c) Water in gasoline
(d) No gasoline in carburetor
(a) Fill tank
(b) Clean bowl
(c) Drain gasoline tank, gasoline strainer and carburetor. Refill with clean gasoline.
(d) Clean fuel line from tank to carburetor. Clean fuel inlet screen in carburetor. Check for clogged vent holes in fuel tank cap.
Battery charge low and does not run engine fast enough Charge or replace battery
Ignition system faulty
(a) Broken distributor rotor
(b) Moisture in the distributor
(c) Condenser shorted or open
(d) Broken distributor cap
(e) Excessively pitted distributor
(f) Points not properly adjusted
(g) Short or open circuit in distributor
(h) Ignition circuit broken
(i) Wet or fouled spark plugs
(j) Cracked or broken spark plug insulators
(k) Ignition switch inoperative
(a) Replace rotor
(b) Remove cap and rotor and dry off. Use compressed air to remove moisture from the distributor.
(c) Replace condenser
(d) Replace cap
(e) Clean contact terminals with fine sandpaper. Blow all sand out of cap before reinstalling
(f) Readjust points
(g) Correct or replace
(h) Check cable from distributor cap-to-ignition coil and check spark plugs for correct wiring or loose connections.
(i) Remove spark plugs, wipe off moisture and dry plugs. Remove carbon. Reset plug gap.
(j) Replace spark plug
(k) Place a jumper wire across the two ignition switch terminals on the back of the switch. Attempt to start the engine. If engine starts, the switch is inoperative and must be replaced.
Carburetor choked too much Open the choke. Wait a few minutes before attempting again to start engine.
Air intake restricted or exhaust is clogged Service the air cleaner and clean exhaust system.
Missing and Backfiring But Fails to Start
Water in gasoline Drain fuel tank, fuel bowl and carburetor. Refill with clean gasoline.
Air leaks around intake manifold Tighten manifold stud nuts. Replace gasket if necessary.
Improper firing order Check spark plug cables for correct installation at spark plugs and distributor cap.
Distributor not correctly timed to engine Check and adjust timing.
Moisture in the distributor cap Remove cap and rotor and dry thoroughly.
Distributor cap shorting out Check for loose contact terminals or dirt in cracked cap.
Missing or Cutting Out at High Speed
Distributor breaker plate not grounded properly Check ground lead wire and screws.
Primary lead not tightened or partially broken Check primary lead wire and screws.
Weak point spring tension Adjust point spring tension or replace points.
Spark plugs faulty Check plug gap.
Point gap incorrect Readjust point gap.
Low voltage to spark plugs caused by defective coil Replace coil
Carburetor:
(a) Float level set to low
(b) Dirt in main jet
(c) Restriction in filter
(a) Check float level. Reset if necessary.
(b) Clean out main jet. Replace if necessary.
(c) Clean out fuel inlet filter in carburetor.
Poor compression:
(a) Heat gasket leaks
(b) Burned valves
(c) Worn piston rings
(a) Replace head gasket
(b) Grind valves
(c) Replace piston rings
Excessive Pinging - Detonation
Distributor
(a) Point gap incorrect
(b) Spark advanced too far
(c) Fouled spark plugs
(a) Readjust point gap.
(b) Check and adjust timing.
(c) Clean plugs and reset gap.
Carburetor:
(a) Main metering system too lean
(b) Float level set too low
(a) Adjust carburetor.
(b) Check float level and reset if necessary.
Cylinder head not bolted down tight Torque cylinder head bolts.
Engine Does Not Operate Smoothly
(a) Pitted distributor points
(b) Cracked distributor cap
(c) Worn or bent distributor shaft
(d) Worn breaker plate hub
(e) Worn distributor cam
(f) Improper point spring tension
(a) Clean and readjust points or replace points.
(b) Replace.
(c) Replace.
(d) Replace breaker plate assembly
(e) Replace.
(f) Adjust point spring tension or replace points.
Engine Does Not Develop Full Power
Distributor:
(a) Point gap incorrect
(b) Ignition timing is incorrect
(a) Readjust point gap.
(b) Check and adjust timing.
Low voltage to spark plugs caused by defective coil Replace coil.
Fouled spark plugs Clean plugs and reset gap.
Carburetor:
(a) Float level set too low
(b) Choke plates partially restricted
(c) Restricted throttle linkage
(a) Check float level. Reset if necessary.
(b) Reposition choke control linkage.
(c) Repair or replace
Insufficient air to engine Service the air cleaner
Late valve timing Check and adjust timing.
Air leaks around intake manifold Tighten manifold or install new gasket if necessary.
Exhaust restricted Remove restriction.
Lack of compression See "lack of compression" in this chart.
Governor worn out or out of adjustment Refer to governor section of "fuel system"
Loss of Oil Pressure
Low lubricating oil level Add sufficient oil to bring level up to specified mark on level gauge.
Clogged oil filter Change filter element.
Oil leaks See "excessive lubricating oil consumption" in this chart.
Engine oil pressure indicator or line defective Replace.
Worn main, connecting rod or camshaft bearings Replace.
Dirt in oil filter relief valve or relief valve spring broken Clean valve or replace spring.
Oil pump worn Repair or replace.
Oil diluted or not as specified Change oil regularly using correct grade specified in Operators Manual.
Lack of Compression
Valves sticking Clean valve guides and stems. Grind valves if needed.
Worn pistons, rings and cylinder walls Replace pistons, rings and cylinder sleeves.
Defective cylinder head gasket Replace.
Broken valve spring Replace
Smoky Exhaust
Engine overload Reduce load.
Incorrect grade of lubricating oil Use grade of oil specified in Operators Manual.
Worn valve guides, valve stems, pistons and sticky or worn oil control rings Repair or replace.
Distributor not properly timed Check and adjust timing.
Choke not fully open Open choke fully.
Excessive Lubricating Oil Consumption
Oil leaks Check and service where necessary; valve covers, tappet cover plate, crank-case front cover, oile seals at front and rear of crankshaft, oil pan plug, gasket, oil filter and oil pressure indicator tube.
Worn valve guides, piston rings, pistons and clogged oil control rings Replace worn parts.
Over-heated engine Refer to "engine overheats" in this chart.
Excessive oil poured into crankcase Drain oil. Add amount specified in operators manual.
Excessive Fuel Consumption
Distributor:
(a) Point gap incorrect
(b) Ignition timing incorrect
(a) Readjust point gap.
(b) Check and adjust timing.
Low voltage to spark plugs caused by defective coil Replace coil.
Worn or fouled spark plugs Clean plugs and reset gap.
Carburetor:
(a) Float level set too high
(b) Leaking needle or seat
(c) Choke plate not fully open
(a) Check float level and reset if necessary.
(b) Repair or replace.
(c) Reposition choke control linkage.`
Restriction in air cleaner Service the air cleaner.
Engine Noises
Loose piston pin. A sharp rap at idling speed. The pin at fault can be found by short circuiting the spark plugs, one at a time, until the noise stops Repair or replace.
A flat slap, when advancing engine speed under load, indicates a loose piston Replace piston.
A metallic knock when idling and retarding engine speed, but disappears under load indicates worn or loose connecting rod bearings. The bearing at fault can be located by short circuiting the spark plugs one at a time. The noise will disappear when the cylinder with the defective bearing is short circuited Replace worn bearings.
Bearing Failure
Low oil pressure Refer to problem "loss of oil pressure".
Lack of oil Maintain proper oil level. Check for leaks.
Engine runs too hot Keep engine at normal operating temperature.
Loose bearings Install new bearings
Improper lubricating oil Use a suitable oil of non-corrosive type, correct grade and viscosity.
Foreign materials entering engine Use clean oil containers when filling engine with oil and see that all gaskets on the engine are in good condition.
Oil lines clogged Clean all oil passages.
Connecting rod bent Align or install new connecting rod.
Crankshaft out of alignment Straighten or install new shaft.
Valves Sticking
Valve springs weak or broken Install new springs.
Gummy deposits from inferior fuel or oil Clean and use proper fuel or oil.
Valve stems scored or carboned Clean. Install new valves if necessary.
Insufficient clearance between valve stem and guide Ream guides for proper clearance.
Overheating Due to Restricted Air Flow
Bent fan blades Straighten and correct pitch of fan blades.
Trash between radiator fins Clean out thoroughly between radiator fins. Use compressed air.
Bent radiator fins Straighten bent fins.
Fan housing damaged or missing Repair or install new fan housing.
Overheating Due to Poor Condition
Restrictions in radiator core or engine water jacket Drain system. Disconnect hoses and reverse flush the radiator and block.
Low coolant level Fill cooling system to proper level. Check for leaks.
Pressure cap loose - coolant escaping Tighten pressure cap.
Fan belt slipping or loose Tighten if loose. Replace belt if greasy, worn or damaged.
Overheating Due to External Leakage
Defective hoses or connections Replace defective hoses. Check clamps for fatigue and replace as required.
Radiator leaks Remove radiator. Test for leaks and solder.
Damaged gaskets Check for leaks around water inlet and outlet elbows and cylinder head. Replace defective gaskets.
Cracked or warped cylinder head Replace cylinder head and gasket.
Cracked cylinder block Replace cylinder block.
Defective water inlet or outlet elbow Replace.
Overheating Due to External Leakage
Warped cylinder head Replace cylinder head and gasket.
Blown cylinder head gasket Replace gasket. (Check crankcase oil for presence of water.)
Cylinder head cap screws not properly torqued Torque head cap screws.
Cracked cylinder wall Replace cylinder block.
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