Adjustment of the throttle is carried out at the right handlebar with the engine running.
Loosen the locking nut on the throttle cable near the throttle twist grip and adjust the throttle by turning the knurled adjusting nut. The correct adjustment is reached when the engine runs smoothly at low engine speed. If the engine is too fast, the Dimoby clutch will automatically engage.
Remove both side covers and start the engine and let it run until warm. Turning the idle adjustment screw to the right (clockwise) will accelerate the engine idling speed, to the left or counter clockwise will decrease the speed.
The correct engine idling speed is reached when the engine is running smoothly with the throttle grip rolled fully away from you. As with the adjustment of the throttle, the engine idling speed must never be so fast as to engage the Dimoby clutch.
Motobecane Mopeds manual Models 7, 50 and 50V
"Throttle adjustment" page 20
"Carburetor Adjustmet" page 20
Maybe this will help get some of those Mobys to idle correctly. Assumptions,
you have cleaned the carb (if needed), air filter, fuel petcock and have a clean supply of the proper gas/oil mix.
Proper gas mix is 4 ozs, 2cycle motor oil, to one gallon of regular gas. All models have a 1 gallon tank except the model 7 which holds .7 gallons.