4.18
CARBURETION SYSTEM
TROUBLESHOOTING
Fuel Starvation / Lean Mixture
Symptoms: Hard start or no start, bog, backfire, popping
through intake / exhaust, hesitation, detonation, low power,
spark plug erosion, engine runs hot, surging, high idle, idle
speed erratic.
•No fuel in tank
• Restricted tank vent, or routed improperly
•Fuel lines or fuel valve restricted
•Fuel filter plugged
•Carburetor vent line(s) restricted
•Plugged or restricted inlet needle and seat screen or
inlet passage
•Clogged jets or passages
• Float stuck, holding inlet needle closed or inlet needle
stuck
• Float level too low
• Fuel pump inoperative
•Air leak at impulse line
•Restricted impulse line (kinked, pinched)
•Intake air leak (throttle shaft, intake ducts, airbox or air
cleaner cover)
•Ruptured vacuum slide diaphragm, Vacuum slide stuck
closed or sticky
•Improper spring
•Jet needle position incorrect
• Incorrect pilot screw adjustment
Rich Mixture
Symptoms: Fouls spark plugs, black, sooty exhaust smoke,
rough idle, poor fuel economy, engine runs rough/ misses, poor
performance, bog, engine loads up, backfire.
•Air intake restricted (inspect intake duct)
•Air filter dirty/plugged
• Choke plunger sticking, incorrectly adjusted choke
• Choke cable binding or improperly routed
•Incorrect pilot air/fuel screw adjustment
•Faulty inlet needle and seat
• Faulty inlet needle seat O-Ring
• Float level too high
• Poor fuel quality (old fuel)
• Loose jets
•Worn jet needle/needle jet or other carburetor parts
• Dirty carburetor (air bleed passages or jets)
•Weak or damaged vacuum piston return spring
•Fouled spark plug
Idle Too High
• Idle adjusted improperly/idle mixture screw damaged
•Sticky vacuum slide
• Throttle cable sticking, improperly adjusted, routed
incorrectly
• Choke cable sticking, improperly adjusted, routed
incorrectly
•Plugged or restricted idle jet
Idle Too Low
• Choke cable bending or incorrectly adjusted
•Idle speed set incorrectly
•Idle mixture screw misadjusted or damaged
• Belt dragging
• Ignition timing incorrect
•Worn jet needle/needle jet
•Plugged or restricted idle jet
Erratic Idle
• Choke cable bending or incorrectly adjusted
•Throttle cable incorrectly adjusted
• Air leaks, dirty carburetor passages (pilot circuit)
• Pilot mixture screw damaged or adjusted incorrectly
• Tight valves
• Ignition timing incorrect
•Belt dragging
•Dirty air cleaner
• Engine worn
• Spark plug fouled
•Idle speed set incorrectly (speed limiter)
•Worn jet needle/needle jet
•Plugged or restricted idle jet
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