Z500 wont start....help please!!!

bike was running perfectly but has been sat for a while, then run for about 4 miles, then sat for a few days and run for about 5 minutes then sat for a few days and now it wont start.

the starter motor turns the engine over but it wont fire, after a few minutes it back fires really loudly, does this suggest too much or not enough fuel.

also i took the plug out and the look black and sooty, again does this suggest too rich or lean?

any ideas to get me started?

To begin with, remove one spark plug, reconnect the plug wire to it and ground it to your engine. Try to start it and see if there is a spark. If not, then check the wires to your coils for power and go backward from there until you find the problem.
If you are getting spark, then you must not be getting fuel. Is it possible that you are out of gas? Sometimes even if you have a little gas in the tank it will not flow into the petcock because of grime, etc... Remove the gas line from your tank and see if anything comes out. If so, then follow the gas line to your carbs and work from there.
Black, sooty plugs indicate either too much fuel, not enough air, or weak spark.

Hope this gets you started;)

bigkwak when i remove the main fuel line from the tank nothing comes out, should it? when i remove the bottom pipe and turn the tap to reserve the fuel pours out.

however i have tried tto start the bike on reserve (just in case!)

Black and sooty means there is gas. Gas is what made the plug black and sooty. Buy some new spark plugs and install them after gapping them correctly.

OK... to the petcock. Your answer made little sense to me. You have a VACUUM petcock. It has three positions: ON/RES/PRI - the smaller line connected to your petcock is engine vacuum. When the bike turns over, vacuum is produced. The vacuum actuates a diaphragm that allows gas to flow when the petcock is in the ON or RES position. If you place the petcock in the PRI position (this means PRIME), gas should gravity flow.

OK... you put it in RES and removed the vacuum hose? Gas flowed.

1. petcock diaphragm not functioning as it should
2. you are out of gas otherwise the reserve tap wouldn't be picking it up

Understand what the ON and RES functions mean. You only have one gas tank. The petcock has two pickup taps. One is located on a tube that sticks up in the tank. This is the ON position. Down below that, there is a shorter tube or just a hole in the petcock that is the RES pickup. This means when you are almost out of gas and the line of gas is below the ON pickup, you can switch to RES and the reserve pickup will draw gas. If gas flows when in the RES position but not the ON position, in any circumstance, that means that you are ALMOST out out gas and the line of gas in the tank is lower than the ON pickup but there is still enough for gas to come out of the petcock in the RES position.

To start the bike, change spark plugs (ask about resolving the sooty spark plug problem in an separate thread), fill the tank with gas or put at least two gallons in and put the petcock on PRI after hooking up the vacuum and gas lines. It should start. If it doesn't then there is something else going on...like no spark or the bike is sadly out of tune.