RE: worst engine ever
There doesn't appear to be anything wrong with your MDS, XJet, that was the problem with them, occasionally you'd get a good one!
I note that yours was the rear exhaust version designed for pattern ships so they may were more carefully built than the run of the mill models, suffice it only to say that in my experience of helping others to set up their engines, they were not very reliable. Some would run at high speed and cut out at idle, with others it was the opposite, most would just cut out in the air however carefully you'd adjusted the carburettor, changed plug types or altered the tank height. Sometimes the problem could be cured by changing the carburettor, the Super Tigre was a popular choice on the early models but most owners just gave up on them.
Two further points:
1. Put a good 61 into any 40 sized ARTF trainer and it will go like hell!
2. Your engine would not have passed the noise test at my club!!
Happy Landings,
DD