Gas coming out of the exhaust?

So - I'm in the process of getting my '81 xj650 running really well, and I've noticed another problem - I saw some gas coming out of the back of the exhaust. At least, I think it is gas. Could be oil, as I know there is a bit of a leak around the valve cover.

I began checking the spark plugs, as I think my engine might be running a bit lean. The plugs are new and have only a few hundred miles on them. The left most plug (left as in left side when sitting on the bike) liked kinda white, while the right most plug was black (it looked oil fowled). Could an oil fowled plug not fire, or not fire properly, thus having a cylinder running correctly? Or is there generally a bit of unspent fuel that comes out of the exhaust (which would be a surprise to me).

I'll get some pictures of the plugs, exhaust, and valve covers and post them here tonight. I'm a little concerned about where the oil is coming from, and if that might mean I have worne piston rings or something similar.

 

Any fluid coming out the exhaust pipe came from inside the combustion chamber. If it's fuel you have one or more cylinders not firing, touch the exhaust header and see if any are cold. Be carefull, they should be very hot normally.
The other possibility is just water vapour coming out due to the system "warming up".

Not a normal situation to have any fluid coming out the exhaust pipe.

Worn piston rings should give bluish smelly smoke out the exhaust, poor performance, and hard starting to the engine.
Oil leaking out the valve cover gasket is just an external oil leak, nothing to do with the engine / exhaust system performance.

 

You have some problem with Cyl. # 4 the one on the right. Try exchanging the two plugs from #1 on the left and #4 on the right. Clean the soot from the surface with a rag, but don't worry too much about the black color left on the surface, it will burn off, if the plug is in good working order when it is put into Cyl #1. Now, when you fire up the engine, check & see if all the exhaust pipes are getting hot. Be very careful, they can get really really hot right as they come from the head. Best tool is a laser style remote thermometer.
If they all are running fine, you probably did have a fuel fouled plug. Perhaps a bit too much enrichment at start.
How do the other two center plugs look?
If Cyl #4 now fouls a known good plug, check the plug wire with another known good plug & see if it sparks outside the engine when the plug is grounded. May be a corroded plug wire connection.