If it is on backwards, some Muncies will go over and down.Another common mistake is putting the arm on the tailshaft on upside down. Make sure it's on correctly. Also, if it's using the stock Muncie shifter, was the shifter apart? If so, make sure the gate plate for reverse wan't put in backwards. My uncle accidentially did this on his Corvette and we worked on it for a couple hours untill I spotted the plate. It will go in backwards.