How to measure a scooter intake manifold

The problem it solves

The intake manifold is a part almost nobody thinks about until it splits, and when it does the symptom is not an obvious leak — it is an engine that will not idle, hunts, or runs lean at small throttle openings and fine everywhere else. Then it gets diagnosed as a carburettor problem, and a carburettor gets bought.

This part does two jobs. It carries the mixture from the carburettor to the head, and it isolates the carburettor from engine vibration, which is why part of it is rubber. Rubber perishes, and it perishes fastest at the point where it is stretched over a spigot and clamped.

A split there admits unmetered air. At full throttle that is a rounding error against the air already flowing. At idle it is proportionally enormous, which is exactly why the symptom appears at idle and disappears when you open the throttle — and why it gets blamed on the carburettor's idle circuit.

The four measurements below let you order a replacement without a model number, and the leak test at the end finds the fault in about a minute using nothing you do not already have.

The mistakes this sheet exists to prevent

• Buying a carburettor to fix an idle problem caused by a split manifold.
• Measuring the spigot's inside diameter when the carburettor clamps over its outside.
• Checking for splits only where you can see, when the split is usually on the underside.
• Reusing the original clamp on a new manifold when the clamp has lost its tension.
• Ignoring the port gasket, which is a separate part and seals a separate joint.

How to use it

1. 01Measure the carburettor-side spigot on its OUTSIDE — that is what the carburettor clamps onto.
2. 02Measure the head-side bolt spacing centre to centre.
3. 03Flex the rubber and look at the underside, which is where splits hide.
4. 04Do the spray test with the engine idling, one joint at a time.