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The Scooter Parts Registry

Printable gauge

Carburettor spigot and bore gauge

Two different diameters get confused when buying a carburettor: the bore, which is what the carburettor is sold as, and the spigot, which is what has to physically fit the manifold. This sheet gives you both at true size.

Set scaling to 100% or Actual size. Never “Fit to page”.

Carburettor spigot and bore gaugeActual size. Stand the spigot or manifold on the rings and find the one it matches.0102030405060708090100This bar must measure exactly 100mm. If it does not, reprint at 100% scale.Carburettor bore — the throat diameter the carburettor is sold as3432302826242221191715Spigot / manifold outside diameter4442403836343230282624Mounting bolt spacing — centre to centre, for flange-mount carburettorsLay the flange over a pair so both bolt holes sit inside the circles.42mm46mm48mm50mm54mm58mmMeasure the spigot across its outside diameter, not the rubber manifold it pushes into —rubber distorts under a calliper and will read small.thescooterparts.com — free to print and share. A measuring aid only: matching a dimension does not prove a part fits.

How to use this sheet

  1. Print at 100% and check the calibration bar measures exactly 100mm.
  2. Take the carburettor off. Stand the spigot end down on the right-hand ring stack and find the ring its outer wall sits on.
  3. Look through the carburettor throat and match its opening against the left-hand rings for the bore.
  4. If yours is a flange-mount type rather than a push-fit spigot, measure the bolt spacing centre to centre against the scale at the bottom.

What else has to match

Bore and spigot get you a carburettor that mounts and flows roughly correctly. Four other things still have to line up, and none of them can be read off a printed sheet:

  • Choke type. Manual lever, cable, or an automatic electric choke with a two-wire plug. They are not interchangeable without also changing the wiring.
  • Throttle cable fitting. Top-entry screw cap versus side-entry, and the cable end shape.
  • Fuel inlet position and whether the machine runs a vacuum-operated fuel tap, which needs a second spigot on the carburettor body.
  • Jetting. A carburettor of the correct bore from a different machine will physically fit and run badly until the jets suit your engine.

A note on going bigger

Fitting a larger bore than the engine was built around is a common modification and a common way to make a machine slower. A bore too large for the engine drops intake velocity at the low revs these machines actually spend their life at. If you are replacing a worn carburettor, replace it with the same bore unless you are also changing the exhaust, the air filter and the jetting to suit.

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