Is there a manual that you can get on the internet to work on the 1975 Troy-bilt Tiller?

Your best place info you need at yahoo troybilt group site. They have files there.

A simple guess is that the white block that centers the forward/reverse lever is out of adjustment. I did a search on google for a long and very useful thread on gardenweb. I am not sure that this is the one because I am too lazy to read thru it, but I suspect it will be useful for you.

Here is a link that might be useful: Try this.

I think it's the block too. Troy gave a tool to adjust this, but you can do it without. Move the block down, tighten, and engage the forward lever. Does it stay engaged with an easy release to go to neutral? That's what you want. Do not adjust so that there is extreme pressure on the belts.

Also, the ball bearing can erode a groove in this block. Remove the block and grind it smooth and adjust as above.