Rich idle, hot stalling and rising oil level on a carburetted 110 cc commuter
A 110 cc four-stroke commuter with a slide-type carburettor has covered 21,400 km and runs badly at idle when hot: after 8 km the engine stalls at traffic lights unless the throttle is held slightly open, and it restarts only with the throttle wide open. The spark plug is dry and sooty black after idling, but a plug chop at 60 km/h shows a light tan insulator. The idle mixture screw is 4 turns out from lightly seated (book base setting 1.5 turns out), the float height measures 3 mm above specification, the float needle valve seat shows a faint ring mark, and the air filter is the standard element, clean and correctly oiled. The engine oil level on the dipstick is 8 mm above the full mark and smells of petrol; the oil was changed 900 km ago. Fuel consumption has risen from 55 km/l to 44 km/l. The owner uses full choke for 30 seconds every morning, then closes it.
1. Reconcile the sooty idle plug with the normal high-speed plug, and explain why the engine stalls when hot but restarts only with a wide-open throttle.
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The readings are consistent, not contradictory. Idling uses the pilot circuit, drawing fuel past the pilot jet, mixture screw and bypass ports. A float height 3 mm above specification raises the fuel level in the bowl and richens every circuit at low air speed, and the mixture screw 4 turns out instead of 1.5 adds more fuel at idle. Hence the sooty idle plug.
At 60 km/h the main circuit dominates, the main jet is standard, and high air velocity leans the mixture enough to give a tan insulator. Idle richness is no proof of main-jet richness - the plug chop is testing a different circuit.
Hot stalling fits an over-rich idle: when hot, fuel vaporises more readily, the mixture becomes excessively rich, the engine loads up and stalls at closed throttle. It then restarts only with the throttle wide open, which is the classic clear-flood technique of adding air to a flooded engine.
The high float level with a worn needle valve seat is the primary fault; the 4-turns-out screw is either a previous attempt to compensate or a misadjustment, and should not be left as the fix.
2. Explain how petrol reached the engine oil, what the 8 mm rise above full means, and the risks of continuing to run the engine.
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Fuel is bypassing the needle valve and reaching the sump. The float level is high and the seat has a ring mark, so the valve does not seal reliably, especially when the bike is on the side stand or the tap is left on; fuel overflows into the inlet and past the rings. Short trips with a rich idle wash fuel past the rings as well. Eight millimetres above full on the dipstick is roughly 0.4–0.5 litre of extra fluid in a small commuter sump.
Risks: the oil is diluted, so viscosity and film strength fall exactly where the bearings, cam and cylinder need them; wear accelerates. Petrol in oil raises crankcase vapour pressure and can push oil out of the breather into the airbox. A rising level with a petrol smell means the engine should not be run until the cause is fixed. There is also a fire risk if an overfull sump discharges through the breather onto a hot engine or exhaust.
Fix the fuel fault first, then change oil and filter - new oil will be contaminated again if the needle valve or tap still passes. Also check the fuel tap vacuum diaphragm for a leak.
3. Give the adjustment and repair sequence, including how the float height and mixture screw should be set, and the checks that prove the repair.
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Sequence:
- Turn the fuel tap off, drain the bowl, remove and strip the carburettor on a clean tray.
- Inspect the needle valve tip and seat; a ring mark, or a tip that does not seal under light pressure, means replacement as a matched set. Check the float for leaks by shaking it and immersing it in hot water.
- Set float height with the carburettor inverted, needle just closed but its spring pin not compressed, measured with a float gauge or vernier within the specified tolerance (typically 0.5–1 mm).
- Clean the pilot jet and passages with carburettor cleaner and compressed air; confirm the mixture screw O-ring and washer are present and undamaged - a missing O-ring makes the screw ineffective and owners wind it out.
- Set the screw to the 1.5-turn base setting, refit, warm the engine, set idle speed to specification, then turn the screw for the highest steady idle, resetting speed as you go.
- Verify: steady hot and cold idle, clean plug after 20 km of mixed riding, consumption returning toward 55 km/l, no petrol smell in the oil after 500 km, and a dry overflow pipe.