Mmi have a 99 1200 sportster xl runs really rough like not on all cylinders not sure about kind of carb but its stock with bike and black smoke coming out of front cylinder and plug is black like carbon buildup inb frt cylinder

A black plug is normally a carb or injector problem. Make sure that the plug is sparking then look to the fuel system. Does the bike have a single or twin carb/injector set up. If single then there is too much fuel and not enough air reaching the front cylinder. Could be a choke problem or possibly a valve is not working or seated/adjusted properly If twin you need to check the front pilot air screw for blockage and make sure it is adjusted correctly.

The stock carb for this bike doesn't have a drain plug, you have to remove the float bowl at the bottom of the carb. You can check the jets with the bowl off. Also check the intake seals where the intake manifold connects to the cylinder heads.

Very easy to remove and disassemble the carb. Just keep track of every thing you did and where it went. It would be best to get the manual if you can.

It sounds like you may have a vacuum leak on the front cylinder where the intake manifold bolts to the front head. This will cause the rear cylinder to run a bit richer than the front and the lean mixture on the front cylinder will make it spit back through the carb occasionally.

Now, you didn't say what type of carb you're running. If the carb has been changed to an S&S carb, the accelerator pump is adjusted too rich. Go to this site, download the installation instructions, and read the part about adjusting the accelerator pump.
http://www.sscycle.com/instructions/instructionslist.php?x_cat=1

Click on "download" for the Super E Super G carb. Instruction sheet 51-1012

Good Luck
Steve

Take the air cleaner cover and the air filter off the carb and ride the bike in the range that you are experiencing the problem. If it runs better with the air filter off, your carb is probably jetted too rich on the main jet. If it runs worse, it' probably too lean.

If you crank the throttle wide open in high gear the bike starts to pull, now, back off just a bit. If it pulls better when you back off just a bit, the carb is lean.

If your bike sat for that long with fuel in the carb, it's probably gummed up the carb on the inside. The carb needs to be disassembled and cleaned out on the inside. This is job best left to someone who is experienced in this area.

Steve

it might be excessive piston slap,=pistons too loose in bore so loosing compresion.ignition timing too advanced so pinking.carberation too weak,

i agree with you.  i had the same thing happen to me varnish on float caused it to stick in the down position when i turned the gas on it would flood out i had to change my oil to. did you stablize it if not get the carb. cleaned and make sure you fog the cylinders and add fuel stablizer to gas dont run it empty and think its good enough its not.

This could be an indication you have bad rings! I have also seen times when the carb vent is plugged and the bowl will overflow into the intake when the bike is not running. This fuel will then seep between the cylinder and the piston into the crankcase. This caused cylinder wall wash and required honing the cylinder and re ringing the pistons