Not starting backfire flame bang after carb cleaning

[crazydred]

Hi, I recently bought an used Gs500 99 model.
It was running on one cylinder, so I took the carbs apart and cleaned them.

Now it won't start, battery is good. Engine turns easily.
It has a nice  blue spark.
Petcock set to prime. (owner said ON stalls the bike)
New airfilter.

It did a big bang and flame came out of the exhaust followed by small black smoke (puffs).
I got scared, so I am looking for help before getting deaf 

[crazydred]

Well left cylinder only work with an iridium spark plug.
Im using a DR8EIX from my Pegaso..

[lucas]

You should check the level of the fuel in your fuel float bowls.  Here is a tutorial

http://www.gstwin.com/float_height_check.htm

[crazydred]

I will do that. Thanks.