Service and Repair Fuel, Ignition, and Exhaust Systems
Fuel, Ignition, and Exhaust Systems
This domain tests mixture delivery, spark production and timing, and exhaust flow - and how they interact. A fault in one often shows up as a symptom in another.
Core rules
- Fuel system: check tank, petcock or pump, filter, and lines for flow and leaks. On carburetors, set float level first because it affects everything, then pilot screw and idle, then jetting. An intake leak (spray carb cleaner at the manifold - idle changes = leak) causes a lean condition and hanging idle.
- Fuel injection: check fuel pressure, injector pulse with a noid light, throttle position sensor voltage, and clean the throttle body.
- Ignition: spark plug gap commonly 0.6–0.9 mm (follow the specification), correct heat range, cap and coil resistance, pickup coil air gap, CDI, and timing marks. A black sooty plug is rich or the wrong heat range; a blistered white plug is lean or too hot.
- Exhaust: check for leaks at the head pipe gasket and joints - a leak leans the mixture and causes popping on deceleration. A blocked muffler reduces power. Never remove the catalytic converter or muffler on a road bike; it violates emission rules.
- Two-strokes: carbon build-up in the exhaust port and expansion chamber matters.
Worked example
A motorcycle surges at steady throttle and the plug is black. The owner fitted a free-flow air filter. Reasoning: more air without more fuel leans part-throttle mixture, but the black plug suggests the needle was previously richened to compensate. The correct fix is to restore the stock intake, set the float level, and re-jet to the manual's baseline, then test at idle, part throttle, and wide open.
Traps
Adjusting the pilot screw to fix a main-jet problem; fitting a hotter plug to mask a rich mixture; ignoring an exhaust leak that mimics lean jetting; changing jets without first checking float level and air filter.