Unless you have a CO metre, best bet is to have a word with the mot person, if friendly they will be happy to tweak, let you tweak the co, from memory I think it is counter clockwise for leaning, clockwise for richening, but readings on gas metre will soon tell you, don't forget to clear the exhaust every 30 seconds or so, hold at 2500rpm for 20 seconds to do this.
Make sure the fan is not on, esp if you have a cone filter, ingesting hot air will raise the co, but let the fan come on then go off so that you know it is 100% warmed up, if way way off scale maybe fit a new ect sender first ?
tabetha