Find an intake air leak

Home mechanic (basic tools) · Time: 20 to 45 min (estimate) (estimate)

Between the carburettor or throttle body and the cylinder head sits a rubber intake manifold (also called the inlet rubber or insulator). Air that enters through a crack here, or through a loose clamp or an open vacuum pipe, has not passed the jets or been measured by the sensors. The mixture goes lean (too much air). Typical signs: idle too high or hunting, revs that hang, popping, a white spark plug and an engine that runs hot. The K-Light 125 manual lists a cracked throttle body insulator and a loose throttle body nut as causes of unstable or high idle.

Safety

• Run the engine outdoors only, and keep hands and the spray away from the hot exhaust.
• Use plain water for the spray test. Never spray carburettor cleaner, brake cleaner or starting fluid on a hot engine; they can catch fire.

Steps

Look first (engine off, cold)

1. Remove side covers or the tank if needed to see the manifold all the way round.
2. Bend the rubber gently with your fingers. Look for cracks, especially on the underside and at the bends. Old rubber goes hard and splits.
3. Check both clamps or the mounting nuts are tight, and that the carburettor or throttle body sits straight and fully in the rubber.
4. Check every small vacuum pipe on the manifold or carburettor: for a vacuum fuel tap, a MAP sensor (it measures intake pressure on injected bikes), an emissions valve, or a blanking cap. A pipe that has come off or split is a leak. A missing blanking cap is a leak.
5. Check the air box rubber between the filter and the carburettor. A leak here lets in dirty air but usually does not upset idle.

Spray test (engine running, warm)

6. Warm the engine and let it idle. Note the idle speed.
7. Spray a fine mist of water at one joint at a time: manifold to head, manifold to carburettor or throttle body, each vacuum pipe end.
8. If the idle speed changes or stumbles while you spray one place, water is being drawn in there: that is your leak.
9. Stop the engine. Replace the cracked part or tighten the clamp. Fit new O-rings where the manifold bolts to the head.

Two-strokes

10. A two-stroke can also suck air through a worn crankshaft seal or a leaking crankcase joint. This gives a high, hanging idle and can seize the engine. It cannot be found with a spray at the intake; it needs a crankcase pressure test at a workshop.

Check your work

• Idle settles at the correct speed and stays steady when you spray the joints.
• Revs fall back cleanly when you close the throttle.
• On injected bikes, an O2 or MAP fault code (if any) does not come back after clearing. See Read engine fault codes.

If it did not fix the problem

• Idle still high: check throttle free play and the choke.
• Fuel-injected bike: Clean the throttle body. The K-Light 125 manual also lists too large an exhaust valve clearance as a cause of high idle: Check and adjust valve clearance.