Two-Stroke Engine Troubleshooting

A little knowledge about the carburetor and a few simple solutions and troubleshooting tips can go a long way when your engine won’t start.

For discussion, we’ll troubleshoot a single-cylinder internal combustion engine and carburetor of your motorcycle.

How a Two-stroke Engine Works

The internal combustion engine of your motorcycle consists of a piston assembly moving in a similar-sized cylinder.

The piston is connected to a crankshaft and flywheel to convert reciprocating motion of the piston to rotatory motion at the rear wheel.

The piston moves up and down in the cylinder and has two positions, TDC (Top dead center) and BDC (Bottom dead center).

The engine generates power by performing the 4 steps:

1. Intake of the air-gasoline mixture.
2. Compression.
3. Combustion.
4. Exhaust.

This constitutes one combustion cycle, and repeats until the engine is shut off.

In a four-stroke engine, one combustion cycle is completed in 4 strokes of the piston.

In a two-stroke engine, the combustion cycle is completed in two strokes of the piston.

The two-stroke engine has an exhaust port and an inlet port on the cylinder. The inlet port is connected to the carburetor.

The piston moves from BDC to TDC compressing the air-gasoline mixture drawn earlier and creates a negative pressure in the crankcase.

Due to the negative pressure, a fresh air-gasoline mixture is sucked by the engine through the carburetor and the mixture enters the engine’s crankcase.

When the piston reaches TDC, a spark from the spark plug ignites the compressed mixture and the force generated, pushes down the piston towards BDC.

While moving from TDC to BDC, the piston opens the exhaust port and allows the burnt fuel to move out of the cylinder. It also pre-compresses the air-gasoline mixture in the crankcase.

As the piston moves down it closes the inlet port and the air-gasoline mixture in the crankcase enters the cylinder through the transfer port and in the process pushes out the exhaust gases.

The piston reaches BDC, moves up and while doing so once again opens the inlet port and compresses the air-gasoline mixture ready for spark ignition.

This cycle repeats on and on.

How the Carburetor Works

The carburetor is a precise and sensitive instrument designed to meet the varying and dynamic needs of the air-gasoline mixture of the engine like at starting, idling, and at different throttle positions.

The inlet of the carburetor is connected to an air filter and outlet to the engine’s inlet port.

The negative pressure created in the engine cylinder and the crankcase influences the airflow into and through the carburetor.

All carburetors, whatever the design may be, are based on a common principle of fluid mechanics, called venturi.

A venturi is a simple tube with convex internal bore and the air passing through it faces a gradual reduction in the internal diameter (bore) up to a point, after which the bore returns to its original size.

Due to this reduction in the bore, the velocity or speed of the atmospheric air passing through the venturi gradually increases reaching its peak at the neck and again decreases gradually.

The pressure exerted by air in conditions like these is inversely proportional to its velocity. An increased velocity at the neck means a reduced pressure. The pressure of air at the neck will be less than the atmospheric pressure.

The neck of the venturi is connected to the float bowl of the carburetor under it, through one or more jet tubes, and the bowl contains gasoline.

Since the gasoline in the bowl is at atmospheric pressure, the low pressure in the neck of the venture causes gasoline to rise through the jet tube and mix with the flowing air. The moving air atomizes the gasoline particles and carries them into the cylinder.

Major Parts of a Motorcycle’s Carburetor

Bowl and float assembly: The carburetor is mounted below your motorcycle’s gasoline tank and draws gasoline from the tank under gravity.

The quantity of gasoline in the carburetor bowl is regulated by a float and a float valve. The float valve maintains the pre-set level of gasoline.

Throttle slide with cutaway and jet needle: The throttle slide moves up and down in the venturi tube and regulates the passage of air to the venturi. The movement of the throttle slide is controlled by your twisting or releasing the grip on your motorcycle handle.

A long tapered jet-needle is attached to the bottom of the throttle valve and the jet-needle moves in and out of the needle jet. The needle jet is located directly above the main jet (jets are tiny brass items with precisely drilled holes).

You can see this jet-needle running through the middle of the carburetor if you remove the air filter.

Now, when you release the throttle on your motorcycle’s handle, the throttle slide moves down and closes not only the air passage but also the gasoline flow (since the jet-needle on the throttle slide moves into the needle jet and closes the fuel passage). The reverse of this happens when you pull your throttle.

The jet needle is an important component in the carburetor.

When the throttle slide closes the venturi completely, air entry into the venturi takes place through the cutaway of the slide (a portion of the bottom corner of the slide is removed).

The performance of the engine can be varied by adjustments to jet sizes, jet-needle taper, and diaphragm spring tension.

Carburetor Metering Circuits

Your motorcycle carburetor has five main metering systems/circuits inside it and these are governed by the throttle slide position.

• Pilot circuit.
• Throttle slide with a cutaway.
• Jet needle and needle jet.
• Main jet.
• Choke.

Your motorcycle carburetor pilot circuit has two adjustable parts, pilot air or fuel screw, and the pilot jet. The fuel mixture can be made lean or rich by operating the screw controlling the flow of air or gasoline.

Depending on the design of your motorcycle carburetor, you will either have an air screw or a fuel screw. The pilot jet circuit meets the demand of the air-gasoline mixture for idling and at low speeds with up to 20% throttle opening.

The throttle slide affects the air-gasoline mixture at low speeds (15 to 25% throttle opening). The cutaway on the throttle slide influences the air-gasoline mixture, larger the cutaway leaner the mixture (more air is allowed to pass), and vice versa.

The jet-needle and needle jet affects the supply of mixture during high speed. A thin taper on the jet needle results in a rich mixture whereas a thick taper results in a lean mixture. The jet-needle has grooves and removable clips for adjusting the rate of fuel flow.

The throttle slide with cutaway and jet-needle is active from 20% to 80% of throttle opening.

The main jet controls the fuel flow between 80% and full open throttle. At 75% and above throttle opening, the jet-needle on the throttle slide is almost out of the needle jet and the flow of gasoline is controlled or regulated by the orifice (hole) in the main jet. The larger the main jet size, the richer the mixture.

When you operate the choke on your motorcycle, it chokes or closes the air inlet of the carburetor venturi and supplies a richer air-gasoline mixture for easier starting.

Carburetors are generally one of two types:

• Carburetors with throttle slide.
• Butterfly-valve (constant velocity (CV) carburetors).

In the CV type, pulling the throttle grip moves the butterfly-valve and this movement regulates the movement of throttle slide.

Troubleshooting a Two-Stroke Engine

Opening the carburetor and reassembling or replacing components like jet-needle or jet tubes may appear easy and simple.

However, wrong assembly or assembling with the wrong jet-needle or jet tubes will result in a malfunction of the motorcycle carburetor and may lead to more problems.

Do not attempt to open the carburetor unless you have previous experience and understand the process. It is always good to consult your authorized service mechanic.

Always read your user manual supplied by the manufacturer and follow the guidelines strictly.

Before inspecting your motorcycle carburetor for any issues, check the following points and make sure they are satisfied.

• You have fresh gasoline mixed with the proper ratio of oil in the gasoline tank.
• Check the tank air vent, connecting pipe from the fuel tank to the carburetor and fuel petcock.
• Ensure the spark plug and magneto of your motorcycle engine are working.
• Make sure the throttle grip and throttle cable are properly adjusted.

Problem: The engine does not start

Please go through your user manual and check if the choke is functioning. If the choke is off, put it on and try again. Ensure the fuel petcock is in the proper position.

In cold weather, the choke is required, put the choke on.

In hot weather, the choke may make the air-gasoline mixture very rich. Try without choke.

If the problem persists, check the carburetor bowl and float assembly.

Problem: The carburetor is supplying a lean mixture of air-fuel.

Symptoms may include:

• Mild backfiring in the engine when going down the hill.
• Unsteady or abrupt acceleration.
• Excessive choke during starting.
• White or light grey spark plugs.
• Excessive or white smoke.

Your motorcycle carburetor will have a slow speed (pilot circuit) fuel or air adjusting screw that regulates the air-gasoline mixture at idle and low speeds (when the throttle slide is closed).

Please go through the user manual of your motorcycle carburetor and operate the screws as per the user manual instructions. It is a good practice to listen to your engine sound to judge the correctness of screw adjustment.

Also, the reasons for the above troubleshooting can be:

• Low level of fuel setting in the carburetor bowl and float assembly, resulting in insufficient gasoline drawn.
• Leaking or faulty inlet manifold sealing.
• Clogged jet tubes.
• Check your user manual and correct the fuel level. If the problem persists, check the sealing and jet tubes.  Get the help of an authorized mechanic.

Problem: The carburetor is supplying a rich air-gasoline mixture.

Symptoms may include:

• Fewer miles per gallon of gasoline.
• Sluggish or inactive acceleration.
• Starts in cold without a choke
• A strong smell of gasoline when idling.
• Uneven and irregular running.
• Black or smoke colored spark plug.
• Black smoke from the exhaust.

A dirty or clogged air filter may be obstructing the airflow resulting in less air and more gasoline. Check your manual to open up and clean / replace the air filter

The rich mixture may also be due to fuel level set too high in the float chamber or worn out the float valve needle or overflowing float chamber due to a stuck-up valve assembly.

Go through your user manual or consult your authorized mechanic.

Problem: No gasoline in the carburetor

Check fuel in the gas tank and check its vent valve. Clean it if blocked.

Check the fuel line from the tank to the carburetor and clean or change if needed.

The float assembly and float valve may be stuck and not working. Go through your service manual. Consult your authorized mechanic if necessary.

Problem: Engine flooded with gasoline.

This can be due to the fuel level set too high in the float chamber or worn out float valve needle or overflowing float chamber due to a stuck valve assembly.

Alternatively, you may have no spark.

Problem: Carburetor out of adjustment

This can result in a rich or lean air-fuel mixture.

Symptoms may include:

• Poor engine performance
• Stalling.
• Improper or erratic acceleration.

This may happen when you keep your motorcycle unused for a long period.

The carburetor needs complete cleaning of clogged jet tubes, stuck-up jet-needle, stuck float valve, and gaskets/diaphragms. Replace parts wherever necessary.

Problem: Engine Knocking

The reasons can be:  

• An incorrect air-gasoline mixture.
• Stale gasoline.
• Faulty spark plug.
• Carbon build-up inside the engine.
• Improper ignition timing

Check the gasoline and change it if necessary.

Go through your motorcycle carburetor and correct the adjusting screws position as per the servicemanual instructions.

If the problem persists, carburetor and engine head cleaning may be necessary.

If you intend to keep your motorcycle unused for a long period, it is a good practice to drain out the gasoline from the gasoline tank and the carburetor.