Is My Choke Open or Closed? How to Tell is Revealed Here

Small engine equipment sometimes utilize a device called a choke to assist with starting the engine. In order to quickly grasp how to properly use a choke, it is first necessary to understand the how and why behind them.

Think of an engine as a glorified air pump. In order for the engine to start and run properly, the proper mixture of air and fuel must be ignited by the spark plug to fire the engine. The conditions required to start a cold engine vs. the conditions required to maintain an already started and warm engine differ. This is where the choke comes in. It is termed a choke because you are essentially 'choking' the engine (of air) on start up. When you close the choke, you are closing off the amount of air entering through the carburetor and into the combustion chamber. So, when you pull the starter rope, when the piston travels downward during the intake stroke, to pull in air, with the engine 'choked' and the choke in the closed position, what actually happens is vacuum is created within the carburetor to draw in more fuel into the combustion chamber. The greater amount of fuel is necessary to start a cold engine.

ALWAYS ensure the choke is in the CLOSED position when starting a cold engine

Then, gradually open the choke once running

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Here is an auto-choke machine, where the choke plate is attached to the the thermostat via a linkage. Don't worry whether your machine is equipped with an auto-choke or a manual choke, they all function in the same manner

Most lawn mower manufacturers include on their machines, decals which provide step by step instructions on various starting procedures, such as how many primes are required for a push bulb, etc. The problem is that more often than not, these decals degrade and fade over time, due to sun exposure, wear and tear or, their simply not provided at all. To build on our previous post on how to use a choke, found here: How to use a Choke: Mastering the Choke Function in Small Engine Operation it may be a challenge to know when in fact your choke is opened or closed. Or, how to decipher the meaning behind choke symbols found on your unit.

An easy way to resolve this potential problem is by using the following depictions:

Always move the choke lever to the CLOSED position to start the machine.

Think of a choke as a door; This is the symbol for choke CLOSED

In fact this symbol may be found on your unit and think of the choke plate (or door) as preventing air from passing through a channel into your engine. Although it may seem counterintuitive to close the choke on cold start up, it becomes more clear when you consider the influence of air pressure and areas of high and low pressure. Inside the engine, the space within the cylinder is at a lower pressure than the atmospheric pressure surrounding the outside of the engine. So when you pull the start rope, and the piston moves down on its intake stroke, the lower pressured air inside the engine cylinder causes the higher atmospheric pressure surrounding the engine, to rush in through the carburetor to equalize this difference in pressure. In doing so it creates the necessary vacuum to pull the air and gas mixture necessary to start the engine. So in summary, To start a cold engine move your choke plate to CLOSED position. Your unit may have a symbol similar to the one above as it is universal. Move your choke lever or handle to this closed position, look for something similar to the symbol above.

This depicts a closed choke creating vacuum in an engine, outside higher pressure air is drawn into the lower pressure area within the engine.

This is an actual choke plate (from the viewpoint of looking into the throat of the carburetor) It is in 'closed' or start position

Once your engine has begun running, open the choke gradually. There are partial positions between completely closed, as shown above, partially open, and completely open. After allowing the engine to run for a brief time (depending on outside temperature) approximately 10 seconds should suffice. Next, open the choke completely to run the machine at maximum speed. The enriched air and gas mixture needed for starting, and the vacuum required to pull it into the cold engine is no longer necessary. Thus, the choke plate can remain opened.

This depicts an open choke in the RUN position.

Choke plate held open or in Run position; when looking into carburetor throat

Again, picture the choke as simply a doorway controlling how air and fuel enters your engine. Keep it closed to start, and open it fully to run. Look for symbols similar to the ones shown above and the good news is these principles apply to all small engine applications! Whether your starting a weedeater, snow thrower, chainsaw, edger etc. this rule remains the same. Your now well on your way to starting your unit with confidence!

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