2nd gear pops out I my f3,
My question is what can cause this and will a trans from a f2 work to replace it?

 

Shifting fork is worn thin. Replacing with another tranny isn't what I would do. Just replace the fork. That's what I did

 

You can't use a shift fork from a F2. The pin that follows the shift drum is smaller. You either have a bad shift fork or a damaged shift drum. The shift fork for 2nd gear is close to the edge of the drum and can break a piece off. I fixed a F2 last winter for a customer in the shop.