When this happens it is usually that the kick start pawl is worn. I have the same problem on my '73 cr250m and it is a common problem of hondas of this era, after all the poor old girls are 30 years old. One solution I know some guys have done is replaced the Kick start assembly with one from a mt250, they don't seem to get as much wear. I have just bought an assembly on Ebay and am waiting for it to arrive, when I install it I'll let you know if it solves the problem.
I've installed the assembly, went straight in no probs, and it works fine
