The Chinese 2-stroke bicycle engine is made from the lowest grade aluminum; exhibiting very poor thermal stability, which allows for a lot of blow-by as the cylinder bore does maintain proper bore concentricity as it heats up; with the piston rings unable to properly provide good sealing between ring face and the bore face.
The only thing that can help rectify this situation is the hydrodynamic barrier of 2-stroke oil; being your greatest friend at preventing blow-by.
Although air-cooled specification mineral oil does not like to burn, and also lowers octane rating slightly (both of which are factors that reduce power) the overall gain by using an higher oil/fuel ratio exceeds the previously mentioned contributors to power loss because the piston ring to cylinder wall oil barrier reduces blow-by; thereby maintaining higher cylinder pressure, which yields a net gain in power, but more importantly, an improvement in low and midrange torque, because the lower the rpm in such a situation, the greater the time available for blow-by.
Another important aspect of oil in these 2-stroke Chinese bicycles is thermal mitigation, as it allows improved cooling of hot spots (due to poor metallurgy and poor thermal stability); carrying that heat out the exhaust, instead of having it build up at a localised point; being the starting location for premature engine failure.
In these low cost engines, the third function of 2-stroke oil is lubrication, but the primary function is hydrodynamic sealing, then heat rejection, and lastly the function of lubrication.
When running 50:1 or 100:1, you have just lost the 1st and 2nd functions of 2-stroke oil in the Chinese 2-stroke bicycle engine.
Having said that, if you are just putt-putting around, without the engine being heavily loaded up, and having an optimal air/fuel ratio, i would recommend 40:1, as the engine will not need higher levels of oil to cope with heat rejection or lubricity, and the hydrodynamic barrier is of no great importance in such a circumstance.
But if you are using the bicycle as a work horse, then you need to operate the engine with consideration to the above mentioned information.
http://motoredbikes.com/threads/more-heavy-haul-australian-style.46503/