one of my downpipes doesn't seem to be getting as hot as the others.
it's getting hot but just not as hot as the other three, i've got a spark so that rules out ignition.
could it be carb set up or something more sinister?

It could be mis-firing a little on that pot, or the mixture could be a bit rich and cooling it slightly.

Best bet is to whip the plugs out and have a butchers - they should all be the same colour. A sort of pale biscuity-brown veering towards greyish is ideal. If the plug from the offending cylinder is black then it's too rich on that pot.

Are your valve clearance checks up to date? It might be worth bunging a new set of plugs in when you've gone to all the trouble of getting the old ones out.

had the plugs out and they are a slightly reddish colour, i think this is caused by unleaded fuel.
Switched no 1 and 2 plugs over but it's still running cool (not cold)
the bike seems responsive enough and doesn't have that choppy feel that 4 cyls have when they're running on 3.
there's fuel in the float bowl too.
main jet blocked maybe?

the valve clearances were all within tolerance when you got it m8, my guess is the fuel, or plugs or coils-is it sluggish/or running on 3 when cold or all the time?
might be the tennis ball we put in no2 cylinder when we discovered the piston was missing...........................................................................................................

Similar problem on my old CB200. Try cleaning out main jets - soak in Cuastic soda - and yes have a fiddle with the mixture adjust. Easiest way to do this is allow a high idle speed @3000, and adjust mixture screw in/out until the highest possible RPM is obtained. I can do this with only the one cylinder running as the CB is a twin.

Have a look at the floats/needle valve set up. If these are set incorrectly fuel may not be getting into the bowl and fuel starvation leads to overheating.