Sorry for stuff being a bit jumbled up chronologically, when this post comes out it will actually look back two weeks to when this actually happened, but a lot has happened since and probably even some more will, as I am still head over heels in doing all the legal work to get my sidecar on the road.
With that being said, right after we got the sidecar outside of the workshop my dad asked me, whether we could pull the engine out of "pretty one", so he could attack thirty years of dirt and grime and have the engine on the bench at the same time, so further investigations can take place.
Taking the engine out on a XT 500/600 4 Valve is a pretty straight forward process: There's one mount on the head, a double mount on the front and the swingarm bolt and one bolt below make up the rear mounts.
A small car jack makes things a lot easier and means you could theoretically even do it alone.
And that's the engine out and on the bench for further investigation.
After that we put a rag over the engine and parked the pretty-one somewhat closer to the wall and decided to swap out the stock carb of the Dre-XT-Stück with the one from the "pretty-one", which also mandated swapping around the inlet-rubbers.
And guess what: Nothing. Same issues as before. So it was pretty clear that it wasn't (only) the carburettor that was causing the trouble, but something else. But I did note, that it would pull cleanly for a short while and then stall. So I decided to renew the fuel line and replace it with a longer hose in a sweeping bend.
Let me politely say: F*ck you hose! F*ck you stock carburettor! F*ck you my own hybris for taking a short cut with a short piece of hose and being sure that it would work alright.
With the old Dre-XT-Stück now running like a scalded cat (a bit rich perhaps), we even went as far as to add the missing left side panel (and peel off some of the old stickers!)