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Analysis And Solution on Typical Carburetor Malfunction

Carburetor is a precision mechanical device of motorcycle. But in actual use, its failure rate isn’t low. Quality problem of related parts shorten the service life of carburetor. Such as low cleanliness, which will increase wearout of carburetor parts.

Carburetor is a precision mechanical device of motorcycle. It is as important as the heart for motorcycle engine. From a professional point of view, its failure rate should be very low. But in actual use, its failure rate isn’t low.

There are two reasons:

  1. All the performance of engine is related to carburetor, such as acceleration, transition, fuel consumption and so on. When judging the malfunction of motorcycle, it is easy to mix the failure of other parts to carburetor and mistaken for carburetor fault and replace the carburetor. For example, when filter failure makes impurities clogging carburetor, the driver will usually change a new carburetor so as to eliminate the fault, but that doesn’t solve the fundamental problem.
  2. Quality problem of related parts shorten the service life of carburetor. Such as low cleanliness, which will increase wearout of carburetor parts.

Here are analysis and solution on typical carburetor malfunction.

Starting failure:

According to the standard, if using†50cc carburetor starting enriching unit correctly and start time over 15 seconds after pedal or electric start, the engine can’t keep continuous operation, then this is starting failure.

The cause and solution are as follows:

There is no oil inside the float bowl of carburetor

There is congestion of carburetor inlet channel. Then you should follow the below step:

1.1 Open the carburetor float bowl and check whether the oil needle valve drops when float drops. If the needle valve is still closely combined with the needle valve seat, that is to say, the congestion is caused by the combination bewteen needle valve and valve seat. Then you can apply alcohol or acetone to clean the gasoline gum bewteen needle valve and valve seat. Such failures often happens on those motorcycle which is not used for long time. Especially when the manufacturer of engine or motorcycle didn’t empty the oil inside float bowl of carburetor and there is also a long inventory or sales period, then there maybe this kind of situation and cause the failure of carburetor.

1.2 Remove the float and valve, get fuel from the carburetor inlet and check, if there is no fuel, that is inlet congestion. Then you can use the compressed air blower to make it clean. Beside, inlet congestion also means there is plenty of impurities inside the carburetor. The fundamental reason is the filter so you need to inspect the fuel filter when cleaning the carburetor.

2.Start enriching unit failure

There is a start enriching unit for the carburetor so as to improve its starting performance, there are two main types of†lawn starter enriching unit:

  • Choke valve: this is a simply mechanical device which is usually used on straddle type vehicle, like motorcycle CG125. You can judge its performance by the choke blade, which will move with the choke handle, so there won’t be many failure for this unit.
  • Bypass enriching system: there are many kinds of bypass enriching system, while electric bypass and manual bypass are the most widely used system. Electric bypass enriching system is commonly used for scooter.

The failure analysis and elimination steps are as follows:

2.1 Four to five minutes after starting the motorcycle switch, touch the plasic shell of electric enriching valve, if you feel the heat, that is to say, the circuit is normal. Otherwise, you need to check the circuit, if it is normal at the connect point of enriching valve, that is to say, the valve is damaged and you need to change it.

2.2 Remove the start enriching valve and connect the circuit, observe the plunger movement of valve within five minutes, if the valve extend with the spring continuously, which means the enriching valve is normal. Otherwise, the PTC heating blade is damaged and you need to change the valve assembly.

2.3 Clean enriching channel with compressed air.

3.Low idle speed

Low idle is when the engine can start but can not be stable after a short running and then shut down.


Adjust plunger screw on carburetor, screw clockwise and then the engine speed will increase while screw counterclockwise, engine speed will reduce. Generally speaking, it is fine to make the engine speed 1500 r/min if it is a astride type car or 1700 RPM if it is a scooter.

4.Incorrect starting

There are some usual incorrect starting way as follows:

4.1 Do not use the start enriching unit.

It is usually happened with the user don’t know much about this function of motorcycle. Even using this unit at room temperature, will also greatly improve the starting performance.

4.2 Keep start enriching unit working all the time during starting.

Enriching unit will provide strong mixture to the engine, if it is working all the time during starting, there will be too much strong mixture inside the cylinder, which will make start difficult.

Correct way to use start enriching unit:

If start three or four times and the engine still cannot startFeature Articles, you need to close the†† enriching unit and screw the throttle lever to rise carburetor†air cylinder and then start again.

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Jack Lee is a writer. He regularly writes about fishing and vehicles. Sunday's coming he wanna drive his car, to his friends apartment with a present like a star, forecaster said the weathers may be rainy hard, but he knows the sun will shine for us. He lives with his wife, dog, and cat in West Virginia, USA.

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