MA12 lumpy Idle

The saga continues... Having bought a replacement head (cheers Matt) , stripped and checked it , relapped the valve seats put in new guide seals etc etc and left nothing to chance the engine has a lumpy idle. If I remove HT lead number 3 the engine still sounds the same.
Have checked valve clearances , swapped leads over , replaced plugs. On the road the engine runs really smooth.The only thing I have not done is a compression test but all bores looked OK.Motor is running the ECC controlled carb as the motor is fitted into a PAO. No cat or lambda fitted so its running in open loop mode.
Any ideas what could be causing the lumpy idle? I'm fearing the worst i.e broken rings on number 3 but hope I am wrong.

Would like to fit the standard MA12 carb on it but donlt know how to bypass all the ECC connections as there are so many of them!

Approaching the end of my tether on this one - maybe I should stick to motorcycles ;)
 
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