Is this likely to cause an engine to run really rich?

Rich enough.. to cause it to fail MOT emissions on seriously high CO?
(0.6% when limits are 0.3%)

Car actually seems to run smoothly, idles really well and starts prompto every time.

Only poor performance I can say is, if the car stalls, it will have to turn over at least 10 times to get going again.
And it seems to be really down on power and general *grunt*, when driving normally and agressively.

Also seems to smell of petrol a bit sometimes.. like its leaking (but there are no leaks).

Thanks in advance (no pun intended)

Chris