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Carburetor starting difficulty analysis
2019-10-15 21:13:51

Carburetor starting difficulty analysis

According to the national standard, under the premise of proper use of carburetor starting thickening device, if the pedal or electric starting time exceeds 15 seconds, the engine still cannot maintain continuous operation and is judged as starting difficulty. The reasons for the difficulty in starting and the corresponding elimination methods are as follows.

1, carburetor float room no fuel oil

The carburetor inlet passage is blocked. Analysis and elimination steps are as follows:

Open the carburetor float chamber and check whether the feed needle valve falls with the float as it falls. If the needle valve is not closely combined with the needle seat along with the float movement, it can be judged that the adhesion between the needle valve and the seat causes the oil inlet channel to be blocked. This fault is generally caused by the condensation of gasoline glue between the needle valve and the seat. Clean with alcohol or acetone. This kind of fault often appears on the motorcycle that is not in use for a long time. In particular, the engine factory and motorcycle factory did not release all the gasoline in the carburetor float chamber after installation. If the inventory or sales period is a little longer, the gel-condensation of gasoline will occur, leading to the performance failure of the carburetor.

Remove the float and needle valve, access gasoline from the carburetor inlet pipe, and observe the outflow of gasoline from the valve seat. If there is no outflow of gasoline, the inlet road is blocked, and compressed air can be used to blow in from the inlet pipe.

In addition, blockages indicate that large amounts of impurities are entering the carburetor. The root cause is the failure of the gasoline filter. Therefore, while cleaning the carburetor, it is necessary to check the gasoline filter.

2. Starting thickening device fails

In order to improve the starting performance, the carburetor is specially equipped with a starting thickening device. The motorcycle starting thickening device has two main structures:

Choke mechanism: choke mechanism is a relatively simple mechanical device generally used in astrocker (such as CG125 motorcycle), can be used to pull the choke handle to observe whether the choke plate is moving with it to determine whether it is normal, this device is less fault.

Bypass thickening system: there are many types of bypass thickening system, and the most widely used are electric heating and manual bypass thickening system. Electric heating bypass thickening system is generally used for scooters. The steps of fault analysis and elimination are as follows:

1) 4 ~ 5 minutes after the motorcycle switch is opened, touch the electric heating to start the thickening valve's plastic shell, and if there is heat sensation, the circuit will be normal; Otherwise, the circuit should be checked. If the junction circuit of the thickening valve is normal, the thickening valve is considered damaged and needs to be replaced.

2) during 0-5 minutes after removing the starting thickening valve and connecting the circuit, observe the movement of the thickening valve plunger. If the thickening valve plunger continues to extend with the spring, the thickening valve is normal; Otherwise the PTC heater in the thickening valve is damaged and the thickening valve assembly should be replaced.

3) clean the thickening channel on the carburetor body with compressed air. Manual bypass thickening system is applied on mulan 50 and other models. The steps of fault analysis and elimination are as follows:

(1) unscrew the starting valve joint, flip the thickening handle switch, and observe whether the thickening cable can drive the thickening plunger up and down. If cannot move or the thickening plunger falls off, the thickening cable is broken and needs to be replaced.

(2) remove the carburetor float chamber and observe whether the inner diameter of the starting foam tube hole on the sealing gasket of the float chamber is smaller than the starting foam tube outer diameter due to expansion and contraction. If it is too small, it is necessary to replace the sealing gasket or increase the inner diameter of the starting foam tube on the sealing gasket, generally larger than the outer diameter of the starting foam tube by 1 ~ 2mm.

(3) clean the thickening channel on the carburetor body with compressed air.

3. Low idle speed

The phenomenon of low idle speed is: the engine can start but not stable operation, a moment later, the flame out.

Elimination method: adjust the plunger adjusting screw of carburetor, screw in clockwise direction, and increase the engine speed; Spinning out counterclockwise reduces engine speed. The general engine speed can be adjusted to 1500 RPM (straddle type vehicle) and 1700 RPM (scooter).

4. Incorrect starting method

Incorrect starting method basically appears in the use of starting thickening device. The common incorrect starting methods include:

Do not use starting thickener. This is due to the user's incomplete understanding of the function of the motorcycle, because even if the use of room temperature starting thickening device, will greatly improve the starting performance.

Starting thickening device (for choke mechanism and manual bypass thickening device) is always used during starting. When starting the thickening system works, the engine is provided with a very thick mixture. If the thickening device is used all the time during the starting process, a large amount of thick mixture into the cylinder will drown the engine and make the starting difficult.

The correct way to use the thickening device is: after starting for 3-4 times, if the engine is still unable to run, close the thickening device, and slightly turn the throttle handle to make the carburetor plunger rise and start again.

Unstable idle speed

Idling instability: after engine running for several minutes to warm up the engine, the engine idling speed fluctuation greater than ±100 RPM is idling instability.

Reasons for the instability of idle speed: under the condition of blockage or leakage of the oil passage or air passage of the carburetor idle speed system, the oil supply of idle speed oil system appears thin or thick, which leads to the instability of the engine idle speed.

1. The idle speed measuring hole is partially blocked

Cause: the hole of idle speed is partly blocked, which makes the oil supply lean under idle speed, leading to the phenomenon of idle speed instability.

Elimination method: according to the carburetor cleaning method can be cleaned.

2, idle speed adjustment screw (commonly known as "wind needle") position change

The role of the idle speed adjusting screw is to change the flow section of the idle oil channel or air passage by adjusting the idle speed adjusting screw, so that the carburetor idle speed oil supply can reach an ideal state. Idle speed adjustment screw according to the function is divided into oil (such as CG125 carburetor) and gas (such as mulan 50 carburetor) two kinds.

For carburetor manufacturers: because of the large impact of the idle speed adjustment screw on the engine performance, the carburetor factory before the idle speed adjustment screw after strict testing and has been adjusted to the best position. Therefore, the user is generally prohibited from adjusting the idle speed adjustment screw. After a long time of use, if the idle adjustment screw position does change and cause adverse consequences can be adjusted. There are two ways to find the best position for the idle adjustment screw:

(1) optimal adjustment method

First piston fixed to more often idle is a bit high engine speed, rotating around the idle adjustment screw, find out the highest speed, the plunger position slightly adjust the plunger adjustment screw, lower the engine speed to find the highest speed, so repeated, until one of the highest at the plunger position is equal to the vehicle standard idle speed. For four-stroke engines, the CO concentration is sometimes very high after the best adjustment.

(2) Paris adjustment method

The Paris adjustment method is based on the best adjustment method, which intentionally rotates the idle speed adjusting screw a little bit in the direction of thinning the mixture (at most 1/8 turn of rotation is allowed), then reduces the rotation speed, and then adjusts the plunger to restore it to the original speed. The result of adjustment should make HC value slightly higher and CO value lower. The principle is HC cannot rise overmuch, with CO a bit lower than standard can. If the result of the Paris adjustment law brings CO up to the standard and HC over the limit, it is not allowed. If CO and HC cannot meet the standards at the same time, it means that the carburetor cannot meet the emission requirements without changing the conditions. It can also be seen from here that limiting CO and HC can guarantee the rationality of adjustment. Otherwise, CO will be blindly lowered, the results make HC too high, combustion in an unreasonable state.

If the emission reaches the standard with the best adjustment method, it is better not to use the Paris adjustment method. If HC reaches the standard and CO exceeds the limit, the Paris adjustment method can be appropriately used. If the Paris adjustment method cannot make CO and HC reach the standard at the same time, the carburetor and ignition system should be checked.

3. Gasket or gasket connecting carburetor and engine inlet pipe is damaged

Damage to the connecting gasket or gasket will lead to air leakage, and additional air will enter the engine, making the fuel supply in the idle state lean, leading to the phenomenon of idle instability.

Elimination method: replace the connection gasket or apron can be.

4. The connection bolt between carburetor and engine inlet pipe is loose

Loose connection bolts will also appear leakage phenomenon. Elimination method: tighten it.

One thing to point out is that most of the carburetors currently used on pedals have an electrically heated bypass thickening system. Under the action of this system, the motorcycle has a high idle speed after starting (up to 2200 ~ 2300 RPM), and the engine only drops back to 1500 RPM after the heating bypass thickening system is shut down for 4 ~ 5 minutes. This is a normal phenomenon, not "idle instability" fault. Hope the user is careful not to misjudge.

Transition bad

In the process of speeding up the motorcycle from the start, the carburetor idle oil supply gradually decreased to the main oil supply increased. In order to make the connection between idle oil system and main oil system smooth, a transition oil system is set to ensure the smoothness of motorcycle starting process. Transition bad phenomenon: start acceleration process, with the throttle of the engine speed fluctuations larger or stall.

The reasons and elimination methods for excessive poor performance are as follows:

1. Idle speed measuring hole, idle oil road, main measuring hole and transition hole are partially blocked

Reasons: the partial blockage of idle speed measuring hole, idle oil road, main measuring hole and transition hole causes the oil supply of carburetor to be sparse, resulting in poor transition.

Elimination method: according to the carburetor cleaning method can be cleaned.

2. Foam tube is blocked

Reason: the role of the carburetor foam tube is to promote the mixing of gasoline and air. After the foam hole on the foam tube is blocked by impurities, the mixing effect of gasoline and air will be reduced, the atomization quality will be reduced, resulting in poor transition.

Elimination method: according to the carburetor cleaning method can be cleaned.

3. Poor idle speed adjustment

Reason: during the transition process, the carburetor oil supply mainly comes from idle oil system. If the idle speed is not adjusted properly, the transition performance will be affected.

Exclusion method: adjust according to the above adjustment method of idle speed adjusting screw.

Running out of steam

Lack of power is mainly reflected in the motorcycle's acceleration performance and high-speed performance.

Motorcycle acceleration evaluation has two indicators: start acceleration and beyond acceleration. Its performance index varies with vehicle type and displacement, and the detection method (such as timing of shift and throttle opening speed) is not easy for users to grasp. Because different users of the throttle control speed is different, the acceleration of the feeling is also different. Therefore, when the user feels bad acceleration, the best diagnosis to professional maintenance point. Users can use the following phenomena to preliminarily determine whether their motorcycle is underpowered. Acceleration process obviously felt slower than before, power decline.

The maximum speed of the vehicle decreased, at high speed, the vehicle "development", the exhaust pipe has a shooting phenomenon.

The reasons for the lack of power and the elimination methods are as follows:

1. Idle hole or main hole is blocked

Cause: idle orifice or main orifice plug will cause carburetor oil supply is thin, resulting in insufficient power.

Elimination method: according to the carburetor cleaning method can be cleaned.

2. Idle oil passage, air passage or main oil passage, air passage is blocked

Cause: idle oil channel, airway or main oil channel, airway blockage will cause carburetor oil supply thin or thick, resulting in insufficient power.

Exclusion method: ibid.

3. Abnormal work of starting thickening device

Reason: this fault mainly appears on the bypass thickening device. Failure of electric heating bypass thickening device or poor starting plunger position in the process of starting plunger extension or poor starting plunger position of manual bypass thickening device will result in the starting plunger not falling to the bottom and the engine running poorly due to excessive mixture concentration.

Elimination method: - for carburetor equipped with electric-start thickening device: replace electric-start thickening valve. - for vehicles equipped with manual thickening device: generally, it is caused by excessive friction between the thickening cable and its shell after a long time of use. It can be solved by applying butter or other lubricating oil on the surface of the thickening cable.

4, acceleration pump device oil is not free or blocked (for carburetor equipped with acceleration pump device, such as CB125 motorcycle carburetor)

Motorcycle in the moment of acceleration, because the plunger lift speed is faster, this time will appear oil supply lag, lean phenomenon. For this reason, the carburetor in some models is equipped with an acceleration pump: at the moment of acceleration, extra oil is supplied to meet the needs of the engine to improve the response of acceleration. Reason: the oil passage of the accelerator pump is blocked or the diaphragm of the accelerator pump fails.

Elimination method: the oil channel of the acceleration pump is blocked and the compressed air is used to clean the oil channel of the acceleration pump. If the accelerator pump diaphragm fails, it is necessary to replace the accelerator pump diaphragm.

The spill

Carburetor feed system is a dynamic balancing system. Under the action of the floating oil buoyancy in the floating room, the needle valve is driven to adjust the gap between the needle valve and the seat to control the oil intake, so that the oil level in the floating room of the motorcycle can maintain dynamic stability under various working conditions. Carburetor leakage phenomenon, is the above balance system was destroyed. Carburetor leakage not only increases fuel consumption and affects the performance of the vehicle, but also causes great harm to the safety of the vehicle. Need to be eliminated in time. Causes of carburetor oil leakage and elimination methods:

1, needle valve and valve seat contact surface adhesion foreign matter

Reason: needle valve and seat is to control the amount of oil intake, its sealing requirements are strict, contact surface smoothness is high. If there is a foreign body attached to the contact surface, it will lead to loose seal between needle valve and valve seat, resulting in oil leakage. Foreign matter mainly refers to the impurities in gasoline and condensation gum. Therefore, to avoid such failures, the user should pay attention to regularly clean the gasoline filter and use good quality gasoline.

Elimination method: according to the carburetor cleaning method can be cleaned.

2, needle valve wear

The reason:

1) needle valve in the process of use due to long-term erosion of impurities contained in gasoline and contact with the seat and wear;

2) unbalanced adjustment at both ends of the float buoy drives the needle valve to wear out due to lateral force. Needle valve wear causes leakage due to loose seal with seat.

Exclusion method:

1) replace the needle valve, and the user should pay attention to regularly clean the gasoline filter and use high-quality gasoline;

2) replace needle valve and adjust both ends of float buoy at the same level.

3, float hair card

The reason:

1) the float is immersed in gasoline for a long time to expand and deform in contact with the float chamber wall;

2) the gap between the float pin and the body float pin hole expands after a long period of friction, causing the float to contact the float chamber wall. The float clip prevents the needle valve from returning to its proper position, leading to oil leakage.

Exclusion method:

1) if the float is deformed, replace the float;

2) if the outer diameter of the float pin becomes smaller, replace the float pin. If the hole of the float pin becomes larger, only the carburetor assembly can be replaced.

4, float damaged or immersed in gasoline

Cause: float damaged or immersed in gasoline will make float weight and buoyancy change, resulting in oil level rise, resulting in oil leakage.

Elimination method: replace the float.

High fuel consumption

The level of fuel consumption is an important performance index that motorcycle users are most concerned about. It is also an important performance index of motorcycle. Carburetor as a key part of motorcycle oil supply system, whether the carburetor is in good condition or not is very important to the influence of fuel consumption. Reducing fuel consumption is also the goal of carburetor manufacturers.

General actual driving fuel consumption rule is: two stroke than four stroke high, large displacement than small displacement high, automatic clutch than manual clutch high. In addition, different engine structure, fuel consumption is also different. Specific value should be decided according to specific model. The fuel consumption of two-stroke 50 cars is about 3L/100km, that of four-stroke 70 ~ 100 cars is about 2L/100km, that of four-stroke 125 cars is about 2L/100km, and that of four-stroke 70-125 scooters is about 3.0l /100km. Users can make a rough judgment about whether their car is on the high side of fuel consumption.

Reasons for high fuel consumption and elimination methods:

1. Carburetor leaks oil

See above for the cause of oil leakage and its removal method.

2. Air volume hole of each oil system is partially blocked

Reason: the air volume hole of each oil system is partially blocked, which will lead to the carburetor oil supply is too thick, resulting in the increase of fuel consumption.

Elimination method: according to the carburetor cleaning method can be cleaned.

3. Starting thickening device is not closed tightly

Starting and thickening


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