GL1100, one black spark plug

Welcome to the Forum, chrishunt!!! Does the engine run smooth or does it have a miss? I would 1st try a new sparkplug, then check the plug cap, and then the plug wire & connections - one thing at a time.

Then you might want to arrange for a compression test, thinking possible head gasket problem(s).

If no joy there, you will likely have to go into the carburetor, looking at float level and loose or damaged main jet or jet needle. There's more...but these things are not rare problems on a 31 year old engine. :waving:

Maybe check to see if it is fuel or oil fouled. If you rub an oil fouled plug on the palm of your hand it will be like black paint and very hard to rub off. Fuel fouled is much easier to get it to lighten in color when you rub it off your hand. Most likely fuel though. It can be caused by so many things including weak spark or even a bad plug. I would try flip flopping the plug. If the problem stays at home then go to ignition troubleshooting then carbs. I am thinking carbs though and the black cylinder carb in particular. You didn't say how it ran. Did it seem like it missed on one cylinder? It can be kind of hard to even detect if you don't have a good feel for what is normal for these bikes. They can sound fairly smooth with one dead cylinder.

Mine turned out to be carb problem. Plug was black & sooty

thanks for replies fellas, will eliminate one thing at a time and see what i get. thanks again. it only misfires when cold and then runs quite well if perhaps a bit lumpy but once wound up it smooths out which i take as normal for a flat 4? appidaze regards chris