downpipes red hot

hi all just a follow up thread from my last on it seems that 1 and 4 down pipes are only getting red hot while it is on cold start and the choke is on as soon as the choke switchs off they cool down to the same as the other 2 middle pipes i habe heck the plugs the are all good and the same colour i have no mods on bike apart from slip on exhaust any ideas
thanks nick
 
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