Sorry, you didn't provide sufficient information for any sort of helpful response. I can tell you that you need three things for it to start. Fuel, air, and a properly timed spark. One of these three things is obviously missing. I have no idea which one it is...
Got any background info on the bike? When was the last time it ran, and what was changed since then?
And you can have fuel, air, a properly timed spark and compression, and it still may not start if the reeds are bad, lol.
But with bad reeds, the problem usually gets worse gradually. In the OP's case, we may have an instance of no-start all of a sudden, though he still has spark. Could be a sheared woodruff key on the flywheel? Just spitballing - need more info as julian said.
You guys are mean!
You can have air, fuel, a properly timed spark, compression, a non snapped powerband, good reeds, a fully intact woodruff key, a good flux capacitor, your induction magnifier nut torqued it to 35 ft/lbs and you can even have the exhaust plug removed (thanks for that one sr5bidder) and it still wont start coz the gas is 3yrs old and your kill switch is faulty.
Please supply some symptoms Mr 1990KDX, other than it wont start. What have you tried?
it has fuel, compression 185 psi , spark, air. the gas is about two-three months old. it is 25 degrees during the day so i put heaters next to it for 3 hours and it still would not start .i kick it over about 100 times. i noticed that there is not a gasket on the exhaust pipe and it leaks when i kick it over
Sounds like you may have some condensation in the float bowl or in the gas tank.
I would drain the float bowl and fuel tank, then get some fresh gas and a new spark plug.
Here is my drill for starting a hard to start bike:
Turn the gas on and lay the bike on its side for 10 seconds - you want to see fuel running out the over flow tubes for at least a few seconds.
Pick the bike up and turn the choke on.
Kick it over SLOWLY a few times to prime the cylinder.
Find top dead center by moving the kickstarter gently - it will get slightly harder to move as you approach TDC. Just as the resistance eases, the piston is at TDC.
Put your right hand on the master cylinder so you don't give it any gas (some bikes do like a tiny bit of throttle on cold start, but you don't want any more than that).
Return the kicker to the top and KICK as hard as you can. If no luck, return to TDC and try again. (kicking from TDC lets the piston build up the maximum amount of speed before the next compression stroke).
First things first then. Drain carb and put fresh gas in it (if it's cold it may have condensation in it). I'd probably pull the carb appart and give it a clean too. Making sure the pilot jet isn't blocked etc and also making sure there is no water in it.
When you kick it over I would not expect it to leak out of the exhaust joint unless the exhaust is blocked or restricted in some way. If it has got a bad joint I would expect it to leak when it's running though. I'd be checking to make sure the exhast is not blocked in some way. Also, put a gasket on it.
Oh, and put a new plug in it, no harm in that.
Then start checking the other things mention except the stooopid ones.
1. Woodruff key on fly wheel to make sure the timing is still good.
3. As it's not a newish bike, check to make sure the motor/carb boot isn't perished and sucking in heaps of air.
There's somewhere to start from anyway.
Re: post above DBD wrote that while I was writing this, then I posted it and Dave had said it too. I didn't copy, promise.