Engine problems

My T1 bus broke down with severe backfires 2 weeks ago. It isn't running since then as I can't find the reason. The engine is fairly new, but somehow it never ran really well. I took it to the workshop where I bought it last year to adjust the engine and carburetors, to fix a lose back wheel and to do some TLC. A few month later I got it back with a missing shock bolt, a fixed wheel bearing but still broken wheel hub and still not well running engine! So the problems were basically the same as before after waiting a few month and spending a 4-digit amount of money. I was fairly disappointed, especially when the owner forbid me to ever show up again at his workshop after I asked (for the 4th time in several weeks) to at least get my parts back that were left there! 

So I don't have an engine builder of my trust anymore...again! Next week I will have to take the daily to Bulli Summer Festival in Wesel....again!