Why do I need to adjust the valves on the Alpha moped? The engine of the moped FMB 139 is equipped with a gas distribution mechanism with a camshaft in the cylinder head. The valves are driven by rockers. During engine operation, the valves experience thermal expansion, so there is a small gap between the beam and the valve, which compensates for thermal expansion. If the clearance is insufficient or excessive, serious engine damage is possible.
The negative effect of incorrect clearances builds up gradually and can cause burnout of the valves, destruction of the seat, decreased compression, overheating and deformation of important timing components. The valves on the Alpha moped must be adjusted several times during the season. If the thermal clearances are incorrectly set, a characteristic knocking sound may appear during engine operation, which is also an occasion for valve settings.
Engine of the moped "Alpha" FMB 139
The four-stroke engine FMB 139 is one of the main advantages that the Alpha moped has. A photo of the engine is presented above. This is a very reliable and economical engine of small weight. Its volume can vary from 49 to 125 cm3, and the power can be from 5 to 8 hp. Modifications to this engine may have different cylinder sizes. You can install at home a more or less powerful version of the cylinder-piston group. The engine is equipped with a four-speed gearbox. Above it is the carburetor of the Alpha moped.
Engine timing FMB 139
The engine timing FMB 139 is equipped with a camshaft located in the cylinder head. The valves are vertically and V-shaped and are driven by a rocker arm, which, in turn, is driven by camshaft cams. This system is very similar to the “Moskvich” 412 timing belt, only one cylinder and two valves.
The camshaft is driven by a chain drive. It, in turn, is driven by a gear located on the axis of the rotor of the generator, which is the axis of rotation of the crankshaft.
Timing prevention and its consequences
All timing elements of the FMB 139 engine need periodic and constant verification of the degree of wear. Stretching the timing chain and the wear of its tensioning elements can lead to its breakage, which will require quite troublesome repairs associated with the need to remove the generator rotor, or disassemble the entire crankcase to replace the gears of the oil pump. In addition, an open timing chain can cause valve deformation, which will require replacement and lapping of the valve seats of the new valve plates. Failure of individual timing elements can cause a number of other serious damage.
Signs of incorrect valve settings
If the thermal clearances are too large, this can cause a characteristic knock or ringing that occurs when the engine is running. Loss of compression can also be an indirect consequence of incorrect valve settings. Indirect consequences of incorrect valve settings can be an increase in fuel consumption with a decrease in power due to depressurization of the valves, as well as a loss of synchronism in their operation, when the valves begin to outrun or lag behind the piston cycles.
The device valves and their grinding
The valves on the Alpha moped are made in the same way as for a car. They have a plate and a rod that lengthens due to thermal expansion during engine operation. The normal clearance should be 0.05 mm. When the valves warm up, the thermal gap almost completely disappears. The valve discs fit tightly into the seat, and their edges must be perfectly ground against the edges of the seat. Lapping the valves on a moped is no different from a similar operation for car engines and is done using the same paste manually, or mechanically using a drill. To extract the valves from the cylinder head FMB 139, a special tube puller is used.
Negative consequences due to incorrect valve settings
The pusher on the beam must press the valve, but with an increase in the thermal gap, it starts to hit the valve, which causes a characteristic ringing knock when the engine is running. This can lead to accelerated wear on critical engine components. The valve disc begins to hit harder on the seat, which causes defects on the edge of the seat and valve. At high temperatures, the surface of the edges in places of small deformations begin to burn, cease to fit snugly together. Sealing is lost, incandescent products of combustion break through the valve and destroy the surface of the edges even more. The valve overheats, and in some cases both the seat and the valve begin to break up, small fragments of which fall into the combustion chamber, scratching the piston and cylinder walls.
In the absence of thermal clearance, the exhaust valve is the first to suffer. The cycle of his work is stretched, which leads to overheating and fusion of its edges. If the valve is pulled, it may stop closing altogether. During operation, an increase in the size of the thermal gap usually occurs, which is the reason for the need for regular checks of this value.
Adjustment of valves on the Alpha moped
The valves on the FMB 139 engine are located above and below the cylinder head. To get to them, you need to unscrew the covers. Adjusting the valves on the moped does not require removing the entire head. But you need to put the cylinder at a dead center. To do this, on top of the generator rotor cover there is a hole on top, which, in turn, is closed with a plastic cover. Unscrewing it, you can see the case of the generator, on which the marks are applied. Using the kick starter, you can gently rotate the generator rotor until the “T” mark appears. The same operation can be performed using the wheel. The moped must be placed on the central footboard, put in fourth gear and carefully turn the wheel until the mark “T” appears in the inspection hole.
To precisely control this process, you can remove the cover from the timing chain block, which is located on the left of the cylinder head. There is a dot mark on this star. If this label is in the extreme left position, this will mean a dead center. After installing the cylinder at a dead point, the valves on the Alpha moped can be adjusted. Loosen the lock nut and pusher. Then you need to push the probe 0.05 mm between the pusher and the valve stem, manually tighten the pusher, but so that the probe can be removed. Then the position of the pusher must be strengthened with a locking nut.