I'm not sure about the Enya plug numbers and how they correspond to OS's numbers. My MDS 48 would do the exact same thing with any plug besides an OS A3. (It would run on an OS #8 but would dead stick on rare occasions, mostly when you did a missed approach and hit the throttle quickly) I also have to run the idle mixture as lean as possible and still have it stay running in order to get a decent mid range. I am not sure how they handle MDS's in the UK but the ones delivered to the US are set up for 10 to 15% nitro. (Someone told me about running low nitro in mine because of it being an eastern bloc engine and when I tried it many problems came up so I went back to 10% and all was well again. Later I was told the US bound engines are set up different because of the fuels we tend to run.) It doesn't like little props (It's not a screamer) and I run an APC 11X7 on mine but it will also turn a 12X4 or 5 pretty good. The most important point of my ramble is mine would not run well on any plug other than an OS A3 and on that plug it runs very good. Good luck with yours.

Edited to add: Like Dar says they like Castor in the fuel. I run Omega in mine which is 70/30 blend synth to castor. Also, mine had a lot of pinch when new and took a gallon or so to loosen up and that's when it really started to run sweet. It still has a lot of pinch but not like when it was new when I had to use a hair dryer for the first couple of tanks to start it.