GL1200 1987 running on 3 cylinders

[Pale Rider]

Hi All,My name is Wayne, I am new to forums and not sure if this is the correct place to put this question but if not possibly an administrator could move it to the correct place. That said I have just bought a gl1200 aspencade from a friend of mine who believes it was running correctly. I have had many bikes in the past and believed it was not running as it should. I got it home and after letting the bike cool down for a while started it up and feeling the exhausts next to the heads I could easily tell that what I shall call No. 3 cylinder was cold whilst the other three exhaust pipe were blisteringly hot. No.3 cylinder I am asumming is right side rear. I removed the lower parts of the fairings to access the plug leads and after removing plug cap no3 it made no difference to idling and throttle responce with the engine running. For completeness I replaced plug lead no.3 and removed plug lead no. 1 (right front) the enginge would not idle stablely and died on letting go of the throttle. On the other side of the machine same response with plug no.4 ( left rear). But with no.2 plug disconnected the bike responded as normal. Idled as normal throttle responce as normal and no.3 exhaust pipe getting hot. the bike runs on three cylinders as a preference with every thing connected minus no.3 but with 3 connected and no.2 not connected runs happy on 1,3,4, does any body have an explanation for this. Any help gratetfully appreciated.