How Does A Motorcycle Choke Work?


If you have ever tried starting a cold engine, you will know that it won’t easily start. This is especially true on carbureted engines, like those found on motorcycles. This is where the choke plays its part.

How does a motorcycle choke work? The purpose of the choke is to deliver an enriched air-fuel mixture into the cold engine to help it start. Having a rich air to fuel ratio flowing into the cold engine helps it start when cold. By having more fuel flowing into the cylinder, there is a higher of ignition. Once the engine is warmed up, the choke is no longer needed. 

The specific design of the choke varies depending on the engine manufacturer, but there are some key common principles that all chokes use.

How The Choke Works

First, a plate restricts airflow to the engine upstream from the throttle plate. Restricting the engine may seem counterintuitive since an engine needs air to ignite. However, the air can be enriched with fuel if there is a controlled restriction. The choke causes more vacuum which draws more cold fuel vapors into the low volume of air. 

Second, the fuel has to come from somewhere other than the main carburetor channel. Typically, there is a well of fuel adjacent to the float bowl. Having a separate source for fuel allows the choke to operate without input from the throttle. Also, since the throttle plate is closed, the engine is creating more vacuum which draws more fuel.

As the engine warms up, the enriched mixture becomes less and less critical. The engine will perform better with the correct fuel mixture, so the choke should be gradually backed off as the engine warms. Once at operating temperature, the fuel and air mixture is regulated by the throttle plate and carburetor.

Some carburetors have automatic chokes that ease the choke off as the engine warms. They typically utilize a metal coil that controls the choke plate. Some designs use the warmth of the engine or exhaust to warm the metal coil. The coil expands with heat, spinning and opening the choke.

Some chokes use electricity to heat the coil, which can be controlled a bit more precisely. Some choke designs are tied into the throttle control so the plate will be opened with throttle application.

Signs That The Choke Is Malfunctioning

Here are a few common signs of a malfunctioning choke:

  • Hard starting
  • Excessive fuel consumption
  • Excessive smoke after the engine is running
  • Spitting or popping through the carburetor
  • Engine starts, then stalls

If the engine is having a hard time starting, even when the choke is supposedly in action, this indicates something is wrong with it. This may require some digging, but there could be a few different things going on. Either the choke isn’t closing enough or the fuel is being stopped. Verify that the choke opens and closes and go from there.

A malfunctioning choke will also cause excessive fuel consumption. The whole purpose of the choke is to create an enriched mixture. If the choke is left on or is somehow stuck on, the enriched mixture will continue to be produced. Even over a relatively short period, like 10-20 minutes, fuel consumption is drastically reduced. There should also be an effect on engine performance due to the restricted airflow.

Excessive fuel consumption will also produce excessive smoke out of the tailpipe. This smoke is unburned fuel escaping into the atmosphere. At startup, there may be a bit of exhaust smoke, but a prolonged amount is indicative of a problem. This may also cause backfiring out of the exhaust.

It is also possible to have backfiring occur through the air intake. A backfire through the carb is most likely indicative of a lean mixture. The uneven burn of a lean condition can carry ignition back through the opening intake valve and into the carburetor. This can be caused by the choke opening too early before the engine is warmed up.

If you are having a problem with the engine starting, then immediately stalling, you might be tempted to think that the choke isn’t working. This can be reinforced when the engine stalls right as you open the choke.

However, the fact that the engine runs with the choke on proves that the choke is working. This problem is probably more likely due to a restriction blocking fuel from getting into the carburetor.

If the choke is malfunctioning, it may be due to the linkage being out of adjustment. The cable has threaded adjustments to tune the opening and closing of the choke. Sometimes the cable can be stretched out and then the adjustment won’t work effectively.

There are return springs that will pull against the cable, so a bad cable could keep slipping out of the selected position. Sometimes the cable can stick or even seize making it difficult to operate.

How To Fix Or Replace The Choke

Since the choke is typically integrated into the carburetor, it isn’t usually a replaceable component. It is possible to replace the choke plate and other components, but it requires disassembling the carburetor. So it isn’t likely that you will need to replace the choke. 

However, some components can be replaced. The most common is the choke linkage. This is usually a lever and cable or a spring-loaded plunger. These are the mechanisms by which the rider can control the choke. In the case of an electrically controlled choke, there is a solenoid that can short internally and can also be replaced.

Replacing the choke linkage will typically require gaining access to the carburetor. If your motorcycle has body panels that enclose the carburetor, start by removing those as needed. It may also be necessary to remove the air cleaner depending on the bike design. On some designs, it will be necessary to remove the carburetor. Study up on what your bike needs and make preparations.

The replacement of the cable is usually straightforward. It may require loosening a few threaded fittings on the handle side and the choke side. If you have the replacement part on hand, make sure it is the same length as a cable that is too long or short won’t work.

Once the link is replaced, put any pieces previously removed back on. Test the cable by actuating it before starting the bike. If it appears to be functioning, try starting up the bike and actuating the choke.

Another component that can break is the choke lever arm. Replacing the arm is going to be a similar process to replacing the cable linkage. The depth of the project is dependent on the specific design of your bike’s carburetor, so study up before tearing into it.

How To Make The Choke Last Longer

Using the choke will cause wear and tear, but it will also make you aware of any maintenance that needs to be done. Over time and with not a lot of use, the choke cable can seize up, causing the choke to be unusable. There is also the possibility of build-up around the choke plate that will affect its operation. Fortunately, these are easy to remedy.

A sticking cable or linkage can be corrected with some WD-40 or other penetrating oil. To accomplish this, you will need to gain access to where the cable connects to the choke plate. Spray the cable and actuate the choke to allow the lube to get into the joint. Periodically lubricating these points will help prolong the life of the choke.

Carburetors are always prone to getting dirty. Make sure to keep the air filter clean. If your motorcycle has been having the issues mentioned above, it is a good idea to clean it out.

If you aren’t into disassembling the carburetor and cleaning it, there are spray cleaners available that can help a bit. Some require wiping out the dirt while some products are used while the engine is running so that the dirt can get burned and exhausted.

Otherwise, the choke should be pretty low-maintenance. If it is used often, it may wear sooner, but at the same time, the consistent presence of gasoline can also keep things clean. Make sure that when storing the motorcycle for winter you drain the fuel and make other preparations so that the fuel system doesn’t get gummed up.

Conclusion

Fuel is more difficult to burn when the engine is cold. The design of the choke is such that an enriched air-fuel mixture can be delivered to the combustion chamber in a cold engine condition. Once the engine is running, everything warms up and it is easier for fuel to ignite, and the choke is gradually opened.

Pay attention to your engine’s startup and idle condition as this may be indicative of a choke malfunction. Keep the linkage lubed up for a choke prolonged life.

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