Thanks for the advice: I dump the chinese tech for Japanese state-of-the-art
Ok, after this I decide to return the Tongxin kit and add some $ to get a far more relaible Panasonic based bike. All the problems have vanished & yes with a 250 Watt engine I could run 35kms mostty uphill (+500m) and have 2/5 of the 10 Ah battery still available.
Bottom line: All these kits have cheap electronics and cannot work without massive batteries.