I hope I don't get reamed out for this by anyone that disagrees , but sometimes the phenomena of a motor starting and running backwards just happens ,with no explanation . it happened to me plenty with good engines ,and I just stop running it ,and try starting it again . make sure before you start it you turned the flywheel so the piston is at the bottom dead center .it may even happen again ,but try a couple times and it will eventually go forward ,provided that you didn't really do something wrong re assembling your engine. you've probably figured it out by now if you tried starting it again . I hope you trust the advice it happened to me before a few times. of course I panicked ,but someone in the pits told me the same advice , and not to worry about it .