How to prime a carburetor was a very common question before the vehicle manufacturers began switching over to the more efficient fuel injection systems.
However, there are still a large number of antique and veteran cars around fitted with all manner of carburetors that, from time to time, will need the carburetor primed for the engine to start.
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How to prime a carburetor
Priming a carburetor is a matter of adding more fuel to the air/fuel mix. If the carburetor has a “tickler” on the float chamber, this is pressed a number of times to raise the fuel level in the float chamber so some excess fuel enters the carburetor intake. If there is no tickler, the air filter can be removed and a small amount of fuel can be sprayed or dropped using an eye dropper into the carburetor intake.
What you Need To Know When Primining a Carburetor
Don’t use too much fuel when priming a carburetor. You could flood the engine which means excess fuel not igniting and wetting the spark plugs.
The internal combustion engine cannot run on pure gasoline. The carburetor mixes gasoline with air creating a mist that is then ignited by the spark plugs.
When an engine is cold, it requires less air and more gasoline. When warm, the ratio of gasoline to air is reduced.
Sometimes, depending on certain factors like weather or the length of time a vehicle equipped with a carburetor has been sitting without running, the carburetor has to be primed.
How do carburetors and fuel pumps work together?
To move fuel from the gas tank to the engine requires a fuel pump.
A mechanical pump is operated by a cam that rotates when the engine is turned over by the starter causing a diaphragm to suck fuel from the gas tank and deliver it to the carburetor.
When the starter is engaged, the engine turns over causing the fuel pump to pump fuel to the carburetor.
An electric pump works off the battery.
When the ignition is turned on, a small electric motor causes the pump action.
How the carburetor works
When an engine is cold, the fuel entering the engine needs to be richer. For this reason, a choke was standard equipment on all cars.
In the early days, a choke was manually operated by pulling a knob on the dashboard attached to a cable that connected to a device in the throat of the carburetor called a butterfly.
What this butterfly did was restrict the amount of air being pulled into the engine and, in turn, make the air/fuel mix richer.
As engines run more efficiently at operating temperature, the air/fuel ratio mix when an engine is cold is different from that when hot, thus the need for a choke.
As cars became more sophisticated, the manual choke was gradually replaced by an automatic choke.
This was operated by a steel spring that closed the choke when the engine was cold and gradually opened it as the car warmed up.
In the very early days of carburetion, and before emission control regulations, the carburetor was vented into the air.
Every carburetor has a float bowl.
A small amount of fuel is stored in the bowl that is controlled by a float that shuts off a small valve when the gasoline level reaches its maximum and opens, allowing more fuel to flow in when it drops.
Being vented into the air, fuel sitting in the float bowl can evaporate if the car is left sitting for some time.
When this happens, the float has to be “tickled” by depressing a little button on the top of the float bowl that in effect, primes the carburetor.
In perfect conditions, a combination of a full float bowl and a closed choke is sufficient to start a cold car.
Back in the very early days of automobiles, it was common practice to prime a carburetor by removing the air filter assembly from the carburetor and pouring a small amount of gasoline down the throat of the carburetor. This is not a recommended practice.
Carburetors on 4-stroke and 2-stroke engines
Most cars produced today have 4-stroke engines. The four strokes are:
- Intake. Fuel/air mix is drawn into the engine.
- Compression. The fuel/air mix is compressed.
- Ignition. The fuel/air mix is ignited.
- Exhaust. The burnt gases are expelled.
Small gasoline appliances such as chainsaws, lawn mowers, leaf blowers, dirt bikes, mopeds, and gasoline-powered weed eaters use a 2-stroke engine.
The two strokes are:
- Up-stroke (ignition and compression): The piston goes up, and the fuel/air mix is drawn in, compressed, and ignited.
- Down-stroke (power and exhaust): The fuel is ignited, the piston is pushed down and as it returns the burnt gases are expelled.
2-stroke engines are lighter than 4-stroke engines. However, they are much more polluting than 4-stroke engines and are not nearly as fuel-efficient.
The carburetors on two-stroke engines are not as complex as those on motor vehicles.
Priming a carburetor on a 2-stroke
When attempting to start a cold 2-stroke engine, it may be difficult getting the air and fuel to mix properly as 2-stroke engines use a mix of oil and gasoline, the oil being used as a lubricant.
2-stroke carburetors are not vented to the air due to the nature of their applications.
For example, hedge trimmers and chainsaws may be used in positions that would allow gasoline to leak if there was a venting hole.
The carburetors on a 2-stroke engine have what is called a tickler. This is a small lever that when pressed a number of times, allows more of the oil/gasoline mixture to enter the engine.
Keep in mind, that using the tickler to prime a 2-stroke engine carburetor should only be done in moderation and only when the engine is cold. Too much fuel can cause a flooded engine.
Flooding occurs when too much raw fuel enters the engine. All internal combustion engines run on an air/fuel mixture regardless of whether a 4-stroke or a 2-stroke.
Too much raw fuel will not ignite and in a 2-stroke engine, the spark plug can be dampened and not fire.
Conclusion On How to Prime a Carburetor
When priming a carburetor, always exercise caution. Keep in mind, that internal combustion engines work on an air/fuel mixture and too much fuel can be worse than not enough.
If you flood the engine through over priming, particularly a 2-stroke, you will have to remove the spark, dry it off and leave it our long enough for the excess fuel in the cylinder to evaporate.
Frequently Asked Questions About How to Prime a Carburetor
Does the fuel pump prime the carburetor?
The fuel pump pressurizes the fuel system, causing the carburetor float bowl to fill. The carburetor mixes the gas with air, causing the engine to start and run.
Does a 2-stroke carburetor need more priming in cold weather than in warm weather?
Generally speaking, 2-stroke engines are easier to start in warm weather. Usually, they require more priming in cold weather.