trx 125 trans issues

[xcavater]

I just picked up a 1985 TRX125 with a trans problem.
It goes into 1st gear and reverse fine with no issue.
When you shift into second, you can feel it engage, but as soon as you give it throttle it makes a banging sound. Almost sounds
like the chain is slipping only louder and harder. It's the same for
all gears except 1st and reverse. The shifter feels goofy and it's hard to find neutral.
I'm looking at splitting the case and diving in, maybe it's an external adjustment?

[86 Quad R]

it sounds like its either bent shift forks, a loose shift drum or the dogs are worn on the dog gears.

[xcavater]

thanks