4.16
FUEL SYSTEM / CARBURETION
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
• Intake air leak ( throttle shaft, intake ducts, airbox or
air cleaner cover)
• 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
• Electric choke inoperative
• 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)
POOR IDLE
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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