My throttle cable keeps breaking about a 1/4 inch from the end in the twist grip housing. So the lead end of the cable is left in the throttle tube with about a 1/4 inch of cable. I believe I have a stock housing with a standard after market tube and grip. The after market tube allows the twist grip to roll backwards pass the hole for the cable. This happened on the old throttle cable and a new one I purchased from KDI. The bike did not come with an original throttle but a broken aftermarket one. What I think is happening is the throttle tube is allowing the cable to act as a stop when I let off the throttle and the housing is cutting the cable.
Any recommendations.

My first glance at your tube indicates it is not correct. I have a 71 RT1B that I'm just now restoring and put the throttle together this weekend. My tube doesn't look like yours at all. I confirmed on Babbitts parts fish that my tube and a tube for a 68 DT1 are the same part number. Go out to Ebay or your favorite site to buy parts and get the correct tube: 214-26243 is the part number. I just found two on Ebay and one went on my RT1B. They are out there for about $20. Original Yamaha part. No more broken cables. Good luck.

Without getting too into it just yet--someone has used a push-pull twistgrip in a DT1 housing so it cant stop as it should & is damaging the cable going back past where it should stop. lunch time--later. sympathies though--go buy a complete early one.
I'm a slow type'r, beaten by two minutes lol.

I have some used aluminum throttle tubes in a box of Yamaha enduro parts... not sure if they were aftermarket or came on one of the early models. The ones I remember were plastic.

But that's the problem, as everybody said already, get the correct tube and it will fix the issue.

My DT1B came with an alloy throttle tube . The current ones are plastic .
They probably changed decades ago .

Well that someone was me. I really did not understand the difference other than the additional hole for the pull cable. Now I know, no stop. Love this site and really appreciate all the help and knowledge you all are willing to share.

HVC carries a (probably) reproduction throttle tube :

hvccycle.net/yamaha-throttle-tube-1968-1...els-214-26243-00-00/

Thanks, I have purchased a new throttle tube & I am up and running again.