Sukida roma running problems ?

Hi. My girlfriend has a sukida Roma Sk 50qt -9 and since fitting a full exhaust system the bike had slowly over the last month losing power and has never really ticked over right from when she brought it. It starts fine ticks over for a bit then just dies unless you keep the revs up slightly also when warm seems to not play up as much. I have changed the spark plug and ht cap , but still has a slight miss fire and still cuts out. It seems like it is running rich but can't find any other screw on the carb to adjust the mixture.
The other thing that I haven't tried is adjusting the valves , any help would greatly be appreciated