212cc predator won't start...

I screwed something up...first it ran nice.. Then it ran for a short time before it shut down...now I can't even get it to start..I have never even rode the engine..this all happened while trying to break the engine in..
I had changed the jet,and the air intake..I had a little trouble getting the air filter to fit..so I put the stock filter back on...I still have the upgraded jet installed... Could that be the issue?? ...Thanks for the advice and help...
Fuel, compression and spark. Does it have all 3? Make sure you have good spark. Your on/off switch could be faulty. Try unplugging the yellow oil level low wire also near the on/off switch. If the engine is low on oil or if you tip the engine, then the low oil sensor kills the ignition. Unplugging the sensor temporarily will help diagnose your problem.
Do like was suggested, squirt some gas in the carb or down the spark plug hole and give it some pulls, see if it starts for a few seconds. If it does them you know you have a fuel issue. Where you "adjusting" the rocker clearance for the valves? If you have them to tight you won't have any compression and it won't start.
Explain your break in procedure and perhaps we can help you get it running again.

Hey thanks everybody..sorry it took a while to respond...I will try all of the above if necessary... For the break in I justnadded gas and oil and started it..it ran perfectly...I didn't run. Again for a few days...it. Started right up the second time..after a few minutes it started wanti to cut off..which eventually it did...
I may have left the carb choke open which I've learned since not to do..but I started screwing around with the fuel air mixture screw...why?? I really do t know..I'm almost sure it's a fuel issue..I just don't know were to start..I'll take you guys advice and let you know how it Went..thanks.
How were you screwing around with the fuel air mixture screw? The stock Predator carb doen't have one. The only screw on it is for the idle speed. If you unscrewed that too far, it won't start because the throttle butterfly will be closed.
Take the chain off and hold the throttle open about 1/2 way and give it some pulls with the choke on and off and see if it starts. If it runs but quits when you try to idle it, then the idle screw is to far out. You'll need to screw that in a little at a time until it will idle without holding the throttle.

Let us know what you did and how it works out for you.