Recently bought a parts bike for the engine. The parts bike was in an accident. Ripped off the left foot peg and shifter. I swapped the engine into my bike, and sometimes in order to get it into first gear I have to roll the bike a little. Once it goes in first it goes through all the gears, but I noticed something doesn't feel right downshifting from 4th to 3rd. Don't really know how to explain how it feels. I've had one of these bikes before and know somethings not right. What could possibly be the issue?