An engine firing up in reverse means the low end mixture and sometimes the high end needle are too rich. Start it in the normal direction, get it cleared out by throtlling up a little and then while holding the blades pinch the fule tubing going to the carb. It should run for 5 seconds and die. If it dies right away you're too lean, if it takes longer than 5 seconds you're rich. The best way to check top end mixture is to fly it and check temperature on the back of the head, 200-220F is good. You can use the spit test to test temp, spit on the back of the head, bubbles is good, sizzles or pops off is lean, does nothing is rich. Or you can tie it to a bench and get under it take it to full throttle and adjust the top end needle till it peaks and back of a few clicks.