The old KIPs systems are pretty reliable except the teeth can get worn. If it's on the original parts, there's a good chance they're rounded off and it's hanging up. Or, if you're lucky, it's just gummed up.
It's not that complicated of a system. Once you get all the pieces in front of you, you'll know what's wrong.
That said, I have at least one reason to question the engineering of those things. I had an '88 KX250 that even when the shaft and gears were all lined up properly, the right side valve would not open all the way. It was like it was off a tooth, but when you moved it over a tooth (which was one off where the dot on the gear indicated it should be), it was even worse in terms of not closing.