Honda CH250 can't start after changing the valve, and there is carburetor reverse injection.

Main reasons for oil return of motorcycle carburetor;

1. The combustible mixture is too thin and the combustion speed is too low. The time from the start of combustion to the end of combustion lasts until the start of the intake stroke of the next cycle, and the intake valve opens, so that the flame ignites the mixed gas in the intake pipe and backfires the carburetor. The elimination method is to check whether the whole fuel supply system of carburetor cracked from fuel tank is blocked and whether there is oil leakage.

2. Intake valve failure. The intake valve is stuck open and cannot be closed. The reasons of valve sticking are: valve guide wear and valve stem bending. In addition, the loose valve seat will make the intake valve unable to close normally, and the spark plug will ignite the mixture and ignite the mixture in the intake pipe, which will lead to the backfire of the carburetor.

3. The intake valve failed, resulting in lax closing.

4. The high-voltage line is inserted in the wrong seat, which leads to disordered ignition, backfire of carburetor and shooting of exhaust pipe.

5. The cylinder liner is burnt through. The cylinder gasket between the two cylinders burned through, resulting in the cylinder channeling.

6. Ignition is premature, and the mixture is ignited when the intake valve is not closed.

Analysis of specific tempering details:

1, improper operation

When the car is running, the engine can only work normally if the throttle handle is turned slowly and steadily, so that the throttle opening is gradually increased and the proportion of air and gasoline entering the carburetor mixing chamber is appropriate. If the throttle handle is turned sharply to refuel, the throttle opening of the carburetor will increase rapidly, resulting in a large amount of air flowing through the carburetor at high speed. Because the fluidity of gasoline is worse than that of air, it is impossible to spray the corresponding amount from the nozzle to mix with air in time, thus making the mixture dilute. After being ignited in the cylinder, the combustion speed of this lean mixture is slower than that of the normal mixture, so that the engine continues to burn at the end of the exhaust stroke. At this time, the fresh mixture just entering the cylinder is ignited immediately at the beginning of the intake stroke, and it is always ignited to the carburetor, causing the carburetor to backfire.

2. The ignition time is too late

Due to the different combustion chamber shape, compression ratio, fuel composition and other factors, the prescribed ignition advance angles of various types of engines are also different. If the engine is ignited at the specified ignition advance angle, it can ensure that the engine can generate the maximum pressure in the cylinder at the end of piston compression, and the mixing dare to have enough time to burn completely in the cylinder. If the ignition time is too late, when the piston reaches the top dead center of the compression stroke, the flame will not spread to the sixth corner of the combustion chamber. If the pressure in the cylinder is too late, the combustion time of the mixture will be shortened. At the same time, at the beginning of the intake stroke, the unburned mixture will ignite the fresh mixture that has entered the cylinder and burn all the way to the carburetor, causing the carburetor to backfire.

3. Improper adjustment of carburetor

If the air adjustment screw of the carburetor is not properly adjusted, the position of the main injection needle is too low or the oil level in the float chamber is too low, resulting in too thin mixture, the carburetor will also be put away for tempering.

4, improper maintenance

The oil path from the gasoline tank to the carburetor is not smooth (partially blocked), and the mixture will be diluted due to insufficient oil entering the carburetor, thus causing the carburetor to backfire.

In order to avoid backfire of carburetor in use, first check whether the oil circuit is smooth before the vehicle is used. Generally, unplug the fuel pipe of the continuous carburetor, turn on the fuel switch, see whether the gasoline is full of the fuel pipe and continuously flows out, and check whether the air adjusting screw of the carburetor is properly adjusted. Check whether the ignition advance angle meets the specification requirements, and do not turn the throttle handle violently during use. Generally, by doing the above, the backfire of carburetor can be avoided. If flashback still occurs, we can consider increasing the height of the main injection needle and the oil level in the float chamber to thicken the mixture and prevent the carburetor from flashback.