Carbureted motorcycles utilize a choke to restrict air flow to the engine causing a rich fuel mixture. The choke, under normal operation, is only used when starting the engine. However problems with the carburetor or choke can arise causing starting or running issues.
First we will look at what a choke’s function is and how that relates to the carburetor. Then we will look into the different issues and warnings a choke can tell you about your motorcycle.
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Function of a Motorcycle Choke
A choke is used to help engines start. It’s function is to restrict (choke) air flow to the engine. The reduced air causes a rich fuel mixture which makes it easier for the bike to start. If you’re experiencing issues with starting your motorcycle in cold weather check out our guide here and here for troubleshooting tips.
The choke is a mechanical lever which is attached to the lever. The carburetor determines the air fuel ratio mechanically. This ratio will be adjusted by the carburetor with throttle input from the rider. The choke allows you to manually adjust the air fuel ratio by reducing the air flow to the engine.
The choke is necessary on carbureted bikes because the engine requires different air fuel ratios at different temperatures. This is due to changes in air density and vaporization of the gasoline.
At cold temperatures engines don’t vaporize fuel the same way it would at operating temperature. Utilizing a choke allows you to enrich the air fuel mixture by reducing air flow. This will increase the vaporized fuel in the engine when the engine is trying to start.
After the engine warms up, the choke can be pushed close. This will allow the air fuel mixture to be governed by the carburetor.
How to Use the Choke
Learning to use a choke can sound like challenge to new riders, but its actually very simple. The choke is normally located on the left side of the motorcycle. On the Honda Shadow, it is located between the cylinders as indicated by the red arrow below.
When you go to start your motorcycle you should pull the choke out as shown in the image below.
After the engine warms up, you should push the choke back in so it looks like the first image. That’s really all there is to operating a choke on a carbureted motorcycle.
When Should the Choke Be Used?
The choke should only be used when starting and warming up your motorcycle. You can leave the choke engaged while the engine warms up to operating temperature. It may be necessary to leave the choke on while you ride for the first mile or so to warm the bike up.
Should You Use the Choke to Start Everytime?
The choke is designed as an aide to help start your motorcycle. If you live in a hot climate, it might not be necessary to start your bike with the choke. In fact when it reaches around 75-80°F I rarely have to use the choke to start my Honda Shadow.
You won’t harm your bike by not using a choke and you won’t harm your bike by using a choke to start it. You do not need to use the choke every time you start your bike. After you get a feel for your bike, you’ll be able to tell by the temperature how much choke you need to use.
Also the choke is not an on or off switch. Full-choke provides the maximum air restriction whereas half choke gives less air restriction. In colder weather you’ll need to pull the choke out further than in warmer weather. Don’t forget to adjust how much choke you use based on the temperature.
Does It Hurt The Motorcycle To Ride With The Choke On?
Another common question is whether or not its okay to ride with the choke on. Well if you have to leave the choke on so the bike stays running, you have a problem which we’ll discuss below. However if you accidentally leave the choke on what are the affects?
Thankfully the affects are minimal. Leaving the choke on for a ride should not have any lasting affect on your engine. All it will do is cause your bike to run richer than it normally does. This could reduce your fuel economy for that ride. If you left the choke for months of riding, you could foul the spark plugs due to the rich fuel mixture.
Its not recommended to intentionally ride with the choke on, but doing so once in a blue moon is nothing to worry about.
Do Fuel Injected Bikes have Chokes?
Before we jump into some of the choke related symptoms a motorcycle could have, lets address this common question. The answer is no, fuel injected bikes do not have a mechanical choke.
A fuel injected bike does not have a carburetor. The air and fuel mixtures are determined by the Engine Control Unit (ECU). If the engine wants a rich fuel mixture for starting, it can automatically increase the amount of fuel injected into the cylinders. Fuel injected bikes start much easier in cold weather vs carbureted motorcycles, however a well functioning and tuned carburetor will have no issues with cold starts.
Motorcycle Dies When Choke is Off
Has it ever happened to you where you’re just itching to go for a ride. You take the bike out start it up, and immediately when you take the choke off the engine dies?
This problem can usually be attributed to a clogged pilot jet. What’s a pilot jet you ask? A pilot jet is an orifice in the carburetor which supplies fuel to the engine at low throttle conditions. When this gets clogged, your engine can still run with the choke on since you’re restricting air flow. However once you push the choke in, all that air rushes in and creates a lean fuel mixture. The engine struggles to create power and dies.
Unfortunately the fix usually isn’t as simple as pouring additives into your gas tank and letting it work the clog out. Gasoline contains many cleaning additives these days, and if you regularly gas up your bike, the chances are adding additives won’t help.
The likely fix is going to involve disassembly of the carburetor and thorough cleaning/rebuilding of the carb. Here’s a brief guide from bikebandits on how to clean your carburetor.
