Jumping out of gear

hi can anybody help i have a 90 cc cases with four speed b gearbox ive been told a zirri fourth gear ,ive replaced crucifom witha like for like 50mm but it still wants to jump out of gear if held when pulling away with revs .the cables are tight and to length could it be i need a 51 mm cruciform or would it not work with wrong cruciform in cheers



Just answered another post with a similar problem.


Check this: