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Improper valve timing.
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Clutch slipping.
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Exhaust pipe or muffler obstructed.
LACK OF COMPRESSION
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Faulty cylinder head gasket.
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Insufficient tappet clearance.
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One or more improperly fitted pistons or piston rings.
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Valves not seating properly.
POPPING BACK THROUGH THE CARBURETOR
(This usually indicates a too lean mixture.)
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Dirt in carburetor. (Fuel pump strainer dirty, see FUEL SYSTEM.)
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Water in fuel.
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Air leak at intake manifold connections.
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Incorrect ignition timing.
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Incorrect valve timing.
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Inlet valves holding open.
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Spark plug wires connected to incorrect plugs. Firing order 1-3-4-2.
ENGINE OVERHEATING
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Lack of proper lubrication.
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Stoppage of water circulation, faulty thermostat ot lack of water.
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Slipping fan belt.
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Ignition timing improperly set.