I recently tried starting my 91 gs500e and it cranked and cranked and then it back fired extremly loud.  Black smoke, ash like material, and flames came out the tail pipe!  Is this a sign of too much gas or too little?
I pulled the spark plugs (which were basically new) and they were black.  I then cleaned them up a little and the bike started but then took a crap.  I tried starting it again and it blasted flames out its pipe again.  Could the piston rings be shot since the plugs are black?  If so, is there a way to check them without a compression tester?  Or does the carburetor need to be adjusted?  And if so, how do i go about doing this?

Thanx!!!!!!!!

rich? airfilter is still clean? if it were oil you would have seen blue smoke

It probably started with the blackened plugs (been running on choke too long?). When trying to start ist you flooded the engine and got fuel in the exhaust as well. This got you the backfire. From here things went downhill.

You can't really tell what was the original problem anymore before you blow out that exhaust with a nice ride or by having it sit overnight.

What else was changed when you exchanged the plugs? It seems to run rich, that gives you the black plugs. They may simply be rated to cold.