Same thing happened with me just after installation. The kick back was very strong and felt like damage would occur or that the kick mechanism was somehow out of adjustment. After taking it apart checking the teeth and entire kick start assembly, It turns out nothing was wrong. The added high compression of the 70cc kit and lack of firing resulted in the symptoms you described. The culprit: .... bad battery. No sh**, really, it was a bad battery. Swapped a new one in, fired first kick and never had an issue since. Not saying this is your issue, but one of those things that seem way out of left field and not what you would associate as being the issue. GL