Your Motobecane Mobylette has a knob on the driven pulley that engages and disengages the rear wheel and pedal crank from the bike's engine. The knob is a metal disc with two protruding lobes, and is marked with two arrows and the letters "M" and "V". The "M" stands for "moteur" (French for motor) and the "V" for "vélo" (French for bike).
Motivation for Using the Switch
This switch is an oft-maligned piece of equipment, because it can be difficult for novice users to re-engage the engine. However, it is worth mastering this procedure; and having done so you will find that it is not difficult at all. The first reason for using the switch is for your own comfort. If you are forced to pedal your Mobylette, whether because you've run out of gas or you're crossing through an area where motor vehicles are prohibited, you will find that it is easier to pedal with this switch in the bike ("V") position. This is because the engine, clutch, and driven pulley will no longer be contributing resistance to your pedaling.
Perhaps more importantly, though, you should learn to use this switch because it may protect your engine. That's because the alternative to using this switch is pedaling while the engine is engaged, but not running. In this case, the piston will still be moving up and down inside the cylinder. You can use the decompression lever to reduce the resistance your engine provides, but it will still be higher than if the engine was totally disengaged. And if you're pedaling because you've run out of gas, then your engine will no longer be getting lubrication. Say you run out of gas, and on your journey home you need to roll down a long hill. If you leave the engine engaged while you roll down the hill, even with the decompression lever held in, your engine will be going through many thousands of cycles at high RPMs-- all without any lubrication whatsoever.
If you haven't run out of gas, your engine will be lubricated as you pedal along, but the gasoline it's taking in won't be burned off. Depending on how long you pedal (and thus how much fuel is drawn in), this could lead to poor running of the motor later, from running too rich; or it could lead to a fouled plug, or even unused fuel igniting in your exhaust pipe as it warms up. At the very least, it will waste gasoline.
How the Switch Works
I encourage you to remove your right-side sidecover, if so equipped, so that you can observe the operation of the switch. The switch is linked to a lever with a two teeth on one side, and a notch between these teeth. You can rotate the driven pulley until this lever comes into view. The notch in the lever fits over two adjacent teeth on a gear that shares a hub with the main drive gear. When the knob is turned so that the lever engages this gear, the driven pulley rotates with the gear; when the lever is disengaged from the gear, the driven pulley can spin freely on the shaft. There is a spring-loaded mechanism attached to the lever that gives it some compliance when engaging the gear teeth.
Use of the Switch
Disengaging the engine should be quite easy, assuming everything is in working order. Simply rotate the knob counter-clockwise, in the direction that the arrow next to the "V" points. The knob should move about 180° before it hits a stop. Your Mobylette is now in bicycle mode.
Engaging the engine can be tricky, until you get the hang of it. The mechanism lacks an alignment function to bring the lever and the drive sprocket together correctly, therefore it's not as simple as rotating the knob clockwise (towards the "M") because the chances are slim that the notch on the lever is aligned with two teeth on the gear already. The Mobylette Owner's Manual prescribes the following procedure for engaging the engine:
- Attempt to rotate the knob clockwise for the full 180° with gentle pressure
- If you're able to rotate the knob all the way, then the engine is engaged
- If resistance is felt before you've turned the knob 180°, then turn the knob counter-clockwise again
- Roll the rear tire backwards slightly
- Try again, repeating as necessary
However, I prefer the following technique:
- Slowly rotating the rear tire backwards, apply very gentle pressure as you turn the knob clockwise
- The lever should slip into engagement when the teeth are aligned properly.
This second procedure works better when the bike is on the centerstand, as it requires rolling of the wheel. Turning the wheel slowly prevents you from hurting your hand when the teeth engage, because once they are engaged the knob you are holding will move along with the driven pulley. Once the lever has started to engage, you know the teeth are aligned and you can finish turning the knob without rotating the wheel further.
- Note: Viewing the back of the pulley from the moped's right side with the right hand sidecover off can assist you in engaging the drive.
If your engine switch is unusable for any reason, or if you don't trust the mechanism, removing the drive belt acomplishes the same function as the engine disengage switch.
On some Mobylettes, the driven pulley will not disengage even when the knob is fully in the "V" position. This can be because the spring-loaded compliance linkage has been damaged, or the mechanism has rusted. Take care not to turn the lever too hard when engaging the engine, or you can cause the spring to pop off. If the lever or spring mechanism appear very rusty, they should be cleaned. You will need wide-opening channel-lock pliers to remove the pulley's dust cap, and ring opening pliers to remove the retaining clip. Though not directly associated with the engagement switch, you should periodically grease the pedal shaft through the grease fitting on the end of the shaft on the left (clutch) side of the bike.