Faultfinder

Everything you need to know to get your carb working beautifully

A handy guide to rebuilding your fixed jet carburettor.

The fixed-jet carburettor is a miracle of compromise. A variable-jet design (such as an SU or Stromberg) usually has a single fuel-dispensing jet that’s metered by a single needle. Once it’s set up properly, it should work perfectly across the whole operating range of the engine. A fixed-jet carburettor, on the other hand, has anything between two and five separate systems to make it work.

An idling circuit keeps the motor ticking over, the main jet supplies fuel for running under load and transition holes dispense extra fuel to bridge the gap between the two when you pull away. There may also be an accelerator pump and a full-load enrichment system for added performance and smoothness. Two barrels may be used instead of one as a further enhancement. Less is certainly not more, here.

Each system is simple enough on its own. If something goes wrong, you just need to identify which bit is causing the trouble. When you’ve worked this out, fixing the problem is generally straightforward.

We’ll guide you through some common faults, as well as showing you a few simple skills that will come in handy if you’re new to carburettor tickling. If your running problems persist, bear in mind the old motoring adage: ‘most carburettor problems are not in the carburettor at all’. Scrutinise the coil, distributor, valvegear and fuel supply system before throwing the hapless instrument against a wall.