🛢️Carburettor

How it works

The carburetor is the component that, on motorcycles from before fuel injection (very common up through the 2000s-2010s depending on the brand, and still found on many 2-strokes, small-displacement bikes, and customs), meters and mixes fuel with air before intake. It works on the Venturi effect: air drawn in by the engine passes through a narrowing that speeds up its flow and creates a vacuum there; that vacuum draws fuel from a constant-level bowl through calibrated passages called jets. Several circuits take over depending on engine speed: the idle circuit (with its mixture screw), the main circuit (main jet and needle under the slide) for mid and high range, and the choke/enrichener circuit for cold starting. The fuel level in the bowl is kept constant by a float and a needle valve, much like a toilet cistern. Common failure modes: jets clogged by varnished fuel (after winter storage without draining the bowl), a bowl level that's too high (a worn needle valve that no longer seats, a holed float), which floods the engine, a torn slide diaphragm, a choke stuck open, hardened leaking gaskets, and mixture or synchronization drifting out of adjustment on multi-cylinder engines. Signs: rough idle, hesitation on acceleration, black smoke, hard starting, fuel leak, excess consumption. Most work (cleaning, adjustment, gasket replacement, checking the bowl level) is within reach of a methodical home mechanic, provided you always start from the manufacturer's original specs and work cleanly, away from any flame.

Maintenance tips

• ✓If you're laying the bike up for a long time (winter), drain the carburetor bowl via its drain screw, or run the engine with the fuel tap off until it stalls: fuel that dries out in the bowl leaves deposits that clog the jets.
• ✓Use fresh, good-quality fuel; fuel that's sat for several months varnishes and fouls the calibrated passages. A fuel stabilizer helps for long lay-ups.
• ✓Clean the jets with carb cleaner and compressed air, never with a piece of wire, which would enlarge their calibrated bore and throw off the mixture.
• ✓Check and set the idle mixture screw to the manual's starting value (often a number of turns out from lightly seated), then fine-tune it with the engine warm for a stable idle.
• ✓Check the needle valve and float height when disassembling: a correct bowl level is the key to clean carburetion. Replace hardened gaskets with a fresh kit.
• ✓On a multi-cylinder bike, have the carburetors' synchronization checked with a vacuum gauge: out-of-sync carbs cost performance and cause engine vibration.
• ✓Never rejet a carburetor by guesswork: after changing the exhaust or fitting a performance air filter, a full rejetting is needed; always start from the manufacturer's specs.