A simple test is to see if the offending cylinder spark plug is dry. If so the fuel supply to that cylinder is suspect and thus the carb supplying it must not have been cleaned properly or is re assembled incorrectly. A sticking float needle may be all it is....or a blockage not cleaned properly....usual suspects would be the idle circuit which has some tiny holes just after the venturi...where the carb inlet narrows...or the main jet clogged, needle clip not in, leaking diaphragm, cocked return spring just to name a few.
If the plug is wet then then spark you see may not be as good or even present under compression. For an engine to run three things need to be present....fuel, air, and a spark. Even a poorly tuned or maintained engine will attempt to fire if these three are present. Check, also the wire and cap to the offending cylinder....sometimes they become loose cracked or damaged enough for the high tension energy to ground out to a metal part. If this bike has any resisters in the wiring...used to be used to prevent interference to TV signals...get rid of them....ditto any device on the plug caps.
One extremely simple test is get the motor running and tap lightly on the carb supplying the offending cylinder....if it starts to run then it is very likely a sticking float or needle. you can do the same to the plug cap and wire....beware of the high voltage....whilst it is unlikely to kill it can give a bloody big jolt.
Good luck