Valve adjustment on the Alpha moped. Moped "Alpha" - photo, characteristics

2024 Author: Erin Ralphs | [email protected]. Last modified: 2024-02-19 11:42

Why do I need to adjust the valves on the Alpha moped? The engine of the FMB 139 moped is equipped with a gas distribution mechanism with a camshaft located in the cylinder head. The valves are driven by rocker arms. During engine operation, the valves experience thermal expansion, so there is a small gap between the rocker arm and the valve, which compensates for thermal expansion. Insufficient or excessive clearance may cause serious engine damage.

The negative effect of incorrect clearances accumulates gradually and can cause burnout of valves, destruction of the seat, reduced compression, overheating and deformation of important timing units. Valve adjustment on the Alpha moped should be performed several times per season. If the thermal gaps are set incorrectly, a characteristic knocking sound may appear during engine operation, which is also a reason for adjusting the valves.

Alfa moped engineFMB 139

The four-stroke FMB 139 engine is one of the main advantages that the Alfa moped has. A photo of the engine is presented above. This is a very reliable and economical engine of small mass. Its volume can vary from 49 to 125 cm3, and its power can be from 5 to 8 hp. Modifications of this engine may have a different cylinder capacity. It is possible to install a more or less powerful version of the cylinder-piston group at home. The engine is equipped with a four-speed gearbox. Above it is the carburetor of the Alpha moped.

FMB 139 engine gas distribution mechanism

The timing mechanism of the FMB 139 engine is equipped with a camshaft located in the cylinder head. The valves are arranged vertically and in a V-shape and are driven by a rocker arm, which, in turn, is driven by camshaft lobes. This system is very similar to the Moskvich 412 timing, only there is 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 generator rotor, which is the axis of rotation of the crankshaft.

Prevention of the timing and the consequences of its breakdown

All timing elements of the FMB 139 engine need periodic and constant checking of the degree of their wear. Chain stretchThe timing and wear of its tensioning elements can lead to its breakage, which will require a rather troublesome repair associated with the need to remove the generator rotor, or dismantling the entire crankcase to replace the oil pump gear. In addition, an open timing chain can cause deformation of the valves, which will require their replacement and lapping to the seats of the new valve plates. Failure of individual timing elements can cause a number of other serious breakdowns.

Signs of improper valve adjustment

If the thermal gaps are too large, this may 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 improper valve setting 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 operation of the valves begins to lead or lag behind the piston cycles.

Valve arrangement and lapping

The valves on the Alpha moped are made in the same way as for the car. They have a plate and a rod that elongates due to thermal expansion when the engine is running. The normal clearance should be 0.05 mm. When the valves are warmed up, the thermal gap almost completely disappears. The valve discs are hermetically sealed into the seat, and their edges must be perfectly lapped to the edges of the seat. Lapping of valvesmoped is no different from a similar operation for car engines and is done using the same paste manually, or mechanically using a drill. A special tubular extractor is used to extract the valves from the FMB 139 cylinder head.

Negative effects due to incorrect valve settings

The pusher on the rocker should press the valve, but as the thermal gap increases, it starts hitting the valve, which causes a characteristic ringing knock when the engine is running. This can lead to accelerated wear of critical engine components. The valve disc begins to hit the seat harder, which causes defects on the edge of the seat and valve. At high temperatures, the edges of the edges in places of small deformations begin to burn, they no longer fit tightly to each other. The tightness is lost, hot 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 itself begin to break up, small fragments of which enter the combustion chamber, scratching the piston and cylinder walls.

In the absence of a thermal gap, the exhaust valve suffers first. The cycle of its work is stretched, which leads to overheating and melting of its edges. If the valve is over tightened, it may stop closing altogether. During operation, the size of the thermal gap usually increases, which is the reason for the need for regular checks of this value.

Valve adjustment 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 a moped does not require removing the entire head. But you need to set the cylinder to a dead center. To do this, there is a hole on the top of the generator rotor cover, which, in turn, is closed with a plastic cover. Unscrewing it, you can see the generator case, on which marks are applied. Using a kick starter, you can gently turn the generator rotor until the “T” mark appears. The same operation can be performed using the wheel. Place the moped on the center stand, shift into fourth gear and carefully turn the wheel until the “T” mark appears in the viewing hole.

To precisely control this process, you can remove the cover from the timing chain block, which is located on the left side of the cylinder head. This star has a mark in the form of a dot. If this mark is in the extreme left position, then this will mean a dead center. After installing the cylinder in a dead center, the valves on the Alpha moped can be adjusted. It is necessary to loosen the lock nut and the pusher. Then you need to insert a 0.05 mm probe between the pusher and the valve stem, tighten the pusher by hand, but so that the probe can be removed. Then the position of the pusher must be strengthened with a fixing nut.