Episode #29 – Carburetor vs. Fuel Injection and Petcock Basics

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.

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:

• On
• Off
• Reserve

  The “On” position is self-explanatory – when you set to this position, the fuel flows through the petcock’s fuel straw to the carburetor and then to the combustion chamber in the engine.    Similarly, at the “Off” position, the fuel flow stops. Again, self-explanatory.    But what does “Reserve” mean? Is there a hidden reserve tank somewhere?    No. The “Reserve” setting is a little bit more complicated.   When the fuel level drops below the fuel straw, there will be no more fuel flow – and when this happens, it might seem like you’ve run out of fuel completely.   If this happens, however, don’t despair – that’s where the “Reserve” setting comes in.   When you set the petcock to “Reserve,” the petcock will open a small valve at the bottom of the fuel straw which accesses a small amount of fuel at the very bottom of the fuel tank.   Now your bike’s running again – although only for a few more miles or kilometers – just enough or you to find a filling station and put more fuel into your tank.    If you’d like to know more about the petcock, I’ve included a link in the show notes to a video from Tomos America that shows how the petcock works.    Now that we’ve talked about the petcock and carburetors, let’s move on to fuel injection.    Electronic fuel injection   In 1980, Kawasaki released the Z1000-H1 – the first motorcycle with fuel injection. Since then, electronic fuel injection, or “EFI” has become standard on most modern motorcycles, for several reasons.   For one thing, the fuel efficiency is much better compared to a carburetted bike.   Secondly, carburetors have to be replaced and cleaned frequently-  particularly the pilot jet, which tends to get clogged up.    Carburetors also need to be synchronized every now and then.   And depending on the outdoor temperature or even elevation, your carburetor might perform sluggishly.    Fuel injection however is designed to be reliable – it’s not affected by temperature or elevation, unlike a carburetor.    And third, fuel injection is more precise – meaning that your motorcycle will run better for the most part.    That’s all well and good, but how does it actually work?   The fuel is pumped out under pressure to match the engine speed and operating temperature. To measure the engine’s rotation speed and air mass, fuel-injected bikes use sensors fitted to the engine.   When these sensors detect a change in throttle position, engine speed, or the amount of air that goes into the engine, the sensors will send a signal to the electronic control unit or “ECU.”   Once this information reaches the ECU, it calculates when the fuel injectors need to be open, but also how much fuel it needs.    So where you do find the petcock or fuel tap on a motorcycle with fuel injection? Simple – you don’t, because It doesn’t have one. Only carburetted bikes have petcocks or fuel taps.   As effective as fuel injection is, it too has its downsides – namely one.    Remember what I said earlier about how fuel injection is designed to be reliable? There’s a good reason for that – if the fuel injector breaks down, it’s hard to get it fixed.    Since the system relies on sensors and complex circuit boards, it’s not something you can work on in your backyard with common household tools.   And not only is it hard to fix, but replacement parts like a new fuel pump and or sensor are also expensive.   And those are the differences between a carburetor and fuel injection.   If you’d like to know more about the parts of a carburetor, I’ve included a link in the show notes to a video about it.   And as always, if there’s a topic about motorcycle mechanics and maintenance that you’d like me to cover, send me an email at podcast@dualwheeljourney.com.   Until next time, keep your helmet on and your eyes on the road. Bye!