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An internal combustion motorcycle, moped, or scooter uses either a carburetor or a fuel injector for its fuel delivery system. And if you’re curious about the differences between the two and how they work, this episode is for you.
Tune in to discover:
- How a carburetor and a fuel injector works, respectively
- The 5 parts of a CV carburetor
- What a “petcock”/”fuel tap” is
Hello, and welcome to another episode of 30-minute motorcycling – a podcast for new, aspiring, and returning riders, where you’ll discover something new about how your motorcycle, moped, or scooter works – in less than 30 minutes. A bike with an internal combustion engine needs some way to mix the correct amount of fuel and air for proper combustion – which is done either with a carburetor or a fuel injector. But what are the differences between the two? In this episode, you’ll discover more about how carburetors and fuel injectors work, and also what a “petcock” is. Let’s begin by taking a look at carburetors. Carburetor Carburetors were common on motorcycles in the past. They’re still around today, but mostly for cheap, small mopeds and scooters. There are two main types of carburetors – slide and CV or “constant velocity carburettor.” Since they’re the most common type, we’ll focus on CV carburetors for this episode. A CV carburetor consists of 5 main parts: The float bowl – where the fuel ends up after entering the carburetor The throat – a narrow tube that the air passes through The venturi, a narrow passage that compresses the fuel as it passes through it The pilot jet, which handles the mixture from idle to about 15-20% throttle. The choke, which assists cold starts by injecting more fuel into the carburetor. The main jet, which handles the fuel mixture when the amount of throttle is near wide-open. And finally, in between the pilot and the main jet, we’ve got the “needle jet,” which handles the fuel mixture when the amount of throttle is equal to 20-80%. Petcock Whether you’re at idle or wide open, to control the fuel flow, carburetted bikes have a special switch typically located below the fuel tank, known as a “petcock”, or if you’re in the UK or Australia, a “fuel tap.” But why do you need a petcock or fuel tap? Because a carburetted bike feeds fuel from the fuel tank to the engine with the help of gravity – i.e. the fuel flows from the gas tank to the engine through a hose. Simple, but effective – at least when the bike’s running. But what happens when you shut the engine off? What stops the fuel flow then? The answer is – nothing. The fuel will still flow through to the engine like usual, and eventually, then the gasoline or petrol will flood your engine and eventually leak all over the place. To prevent this, you’ll need something that blocks the fuel flow when you’re not riding your bike and activates it again when you’re about to set off. That’s where the petcock or fuel tap comes in. No matter what you call it, it typically has 3 settings: