Jumping out of second gear

[Bretty]

This photo shows the worn bits. The three dogs on the top of the left gear should have sharp edges. You can see the rim underneath that the fork runs in. The fork pushes that lefthand gear across on the shaft so the dogs engage into the holes/slots in the righthand gear. This should also have sharp, not rounded edges in the holes. The combined wear means under any torque they are forced apart and it jumps out into the next hole.
I thought my bike was misfiring!

It cost me £80 for a donor engine, plus a head gasket, a tube of that liquid gasket stuff and three days to get the engine out/in and switch the gearbox in mine. It would be certainly easier if you can do it in your kitchen!