I agree with the above points. The heli-coil repair is very simple and you can buy the kit at larger auto parts stores. If they don't have it they can usually get it by the next day. Of course there are many online sources than can have the kit in your hands in a few days. WW ... dot -dot, make a few clicks and wait for the postman or brown truck to arrive.

Regarding the now quite old DL50, that was the first engine that DL / DLE produced and yes there were several known problems ..... all of which have been resolved in newer models. Judging DLE by how that engine runs would be like judging a new Ford, Chevy or other car by comparing it to something that brand produced long ago. A completely irrelevant comparison and not a basis for deciding whether a current product is a good one.

And about Zenoah ..... they've never had any problems?

As Zeeb suggested, you have an air leak. A common problem was the early Walbro clones that were being used on the DL50. Many had throttle plates that didn't seat well in the carb bore. More than a few were even loose as received. Some had oversize holes where the throttle shafts go through the carb body. Any of the above can cause the engine to idle poorly or with erratic speed or not be able to shut down when fully closed.