P38 running "fluffy"

Hi all,
New P38 owner so be gentle please........

'97 4.0 man, had liners last yr 100k on the clock.
Starts on the button, no warning lights settles to idle but not smooth, warms up with no overheating.
I bought it with all sorts of exhaust blows fron the O/S manifold so couldn't really hear the "fluffyness" or slight miss.
Replaced both O/S maifold gaskets plus downpipe gasket, still blowing so removed manifold and welded gas tight then refit.
Much quieter but O/S cat sounds like a bag of spanners now i can hear it !
Anyway at this point i noticed a slight missfire (what I'd call running fluffy)
so checked plugs (O/S bank running weaker than N/S)
spark to all plugs ok, cleaned injector connectors, air flow meter, TPS, plenum, and all associated connectors.
Sprayed liberal dose of carb cleaner round manifold with no change.
It's hard to start with the AFM (MAF?) disconnected but if I pull it when its running it makes abolutely no difference to the way the car drives.
I'm thinking it's time to get the codes read and cleared and see if it confirms AFM fault ?
So in light of I'm out of ideas and the gurus hang out here..........
Any other thoughts from the panel please ?

Thanks for taking the time to read.

try looking at it running in the dark look for arcing from any leads..