Why does the exhaust pipe of scooter smoke?

When a four-stroke motorcycle is running, there should be no obvious smoke in the exhaust gas discharged by the engine. If there is a lot of smoke, even black smoke, it means that the engine is out of order, and it should be repaired in time after finding out the reason.

1. Exhaust gas

(1) At this time, the mixture provided by the carburetor is too rich, the gasoline burns incompletely, and the precipitated carbon particles are discharged with the exhaust gas. The reason for this is the following：

The air filter is clogged.

The compensation air passage or idle air passage of carburetor is blocked.

The oil level in the float chamber is so high that some oil directly rushes into the cylinder without atomization, resulting in incomplete combustion.

The main metering hole is seriously worn, which increases the fuel injection quantity of the main nozzle and causes incomplete combustion.

The automatic valve for starting heating is in the open position, and even after the engine is warmed up, the heating valve cannot be automatically closed, resulting in too much mixed gas.

(2) Maintenance methods

First, check whether the air filter is blocked. For paper filter, it is generally best to replace the filter. If foam filter element is used, it should be cleaned in kerosene or incombustible cleaner. After drying, apply a small amount of lubricating oil on its surface (be careful not to apply too much, otherwise the air intake will be reduced), clean the filter wall and put it back in its original place.

After confirming that there is nothing wrong with the air filter, check the carburetor. For carburetor with main throttle, all holes should be blown clean with compressed air. Check whether the position of the main oil needle is appropriate. Generally, the oil needle card should be in the third slot. Check whether the main oil needle is excessively worn. If it is worn, replace the valve seat of the float needle at the same time. If the main measuring hole is seriously worn (usually, the diameter will become larger when the iron wire is used to remove foreign bodies during maintenance), replace it with a new one. Check whether the height of the float is appropriate. The specific method is to turn the carburetor upside down, disassemble the float chamber, make the float tongue press the float needle valve, and measure the distance from the joint surface of the float chamber to the top of the float to make the height of the float consistent. For example, JH70 is 20mm, CG 125 is18.5mm. If the float is not suitable, use needle-nosed pliers to pull the float tongue for adjustment.

In addition, for the vacuum membrane carburetor such as HAMA 125, Honda CH 125 should also connect the yellow line for starting the heating valve to the positive battery and the green line to the negative battery. After about 5 minutes, connect the polyethylene pipe to the fuel increment circuit and blow it with your mouth. At this point, the oil loop is completely closed and there is no ventilation.

If ventilation is convenient, it means that the automatic valve cannot close the fuel increment circuit, and the automatic valve that starts heating should be replaced.

Second, the tail gas emits blue smoke.

At this time, the lubricating oil participates in the combustion in the combustion chamber. The four-stroke engine adopts the lubrication mode of splash and pressure. For the camshaft, rocker arm and connecting rod of engine valve train, lubricating oil is delivered to the surface of moving parts for lubrication through the pressure generated by oil pump. For the lower part of the cylinder arm, when the crankshaft rotates, the small end of the connecting rod and other parts are splashed with lubricating oil under the action of centrifugal force. Because the piston moves in a reciprocating straight line in the cylinder, the piston ring will form a layer of lubricating oil film on the cylinder wall, scraping the excess oil on the cylinder wall back to the crankcase. This lubricating oil film has a certain viscosity, which improves the sealing performance of the cylinder, prevents the gas in the combustion chamber from escaping into the crankcase, and also prevents the engine oil in the crankcase from escaping into the combustion chamber. This is the reason why the fuel consumption of a four-stroke engine is low under normal conditions. However, under the following circumstances, the lubricating oil may enter the combustion chamber for combustion, causing the exhaust gas to emit blue smoke.

(A) the reasons for the piston assembly

Piston rings and pistons are seriously worn;

The piston ring is stuck in the ring groove due to carbon deposition, and the cylinder cannot be sealed;

Piston ring is broken;

Piston rings are installed in opposite directions, or the openings are in a straight line.

1, 2, 3, check the piston diameter when repairing, and replace the piston when reaching the wear limit. Check the clearance, elasticity and light leakage of the piston ring. If it reaches the limit value, replace the piston ring. For the combustion of lubricating oil caused by reason 4, it must be installed according to the requirements of piston rings. The chrome-plated ring (or nitride ring) is installed in the first ring groove, and the carbon steel ring is installed in the second ring groove, with the torus marking face facing upwards, the chamfer groove of the outer circle facing downwards, and the ring mouth is far away from the center of the pin hole and evenly staggered along the circumferential direction. There is no direction requirement for the rain wiper blade of combined oil ring, but the opening of wiper blade should be about 20mm away from the opening of liner ring. Note that the piston ring opening deviates from the piston side pressure direction.

(2) The valve stem and the movement of the valve stem play a guiding role. In order to ensure the flexibility of valve movement, considering the thermal expansion factor, there is a certain fit gap between the valve stem and valve guide of intake valve and exhaust valve. With the increase of motorcycle service time, the valve stem and valve guide will wear, and when the fit clearance reaches the limit, it will enter the combustion chamber for combustion through pressure lubrication. When repairing, measure the diameter of the valve stem with an outside micrometer. If the limit value is reached, replace the valve, which should be ground together with the valve seat. Rotate the reamer to the right before measuring the valve guide. If it is rotated to the left or directly inserted or taken out, the inner surface of the guide rail will be damaged. Measure the inner diameter of the valve guide with an inner diameter dial indicator. If it is larger than the maintenance limit size, replace it.

Valve guides are divided into flanged and flangeless types. Replace the flanged valve guide as follows:

Heat the cylinder head evenly to 100℃~ 150℃ (put it in the oven), and do not use a gas burner for local heating to prevent the cylinder head from deforming.

Knock out the valve guide with a special tool, and be careful not to damage the joint surface of the cylinder head;

Apply lubricant to the new O-ring and install it on the valve guide;

Before the cylinder head cools, insert a new valve guide from the camshaft side.

Hinge the valve guide with a reamer, and the reamer should not be skewed when reaming.