YFZ dies, won't start

I have a 2004 yfz that runs well for about 2 minuets then dies.  If I try to start it right away it won't crank or sounds like the timing is way off.  If I wait awhile it will start again and then die.  Help!  

could be the throttle position sensor.

could be the cdi, could even be that the timing chain is loose.

could simply be the fuel/air mixture is out.

there are just too many possibilities.  
Keep it RUBBER SIDE DOWN!!

We put new piston and rings, adjusted valves but no change  When it dies it sounds electrical as it instantly stops.  Then won't crank.  Remove sparkplug and it cranks fine  Thanks  

not electrical unless the spark plug cap is loose, if you can remove the spark plug and it starts and the spark plug cap isn't loose, then chances are it's flooding.

try turning the fuel/air mixture screw to the right 1/4-1/2 turn.  of course once you have ruled out the spark plug cap.

have you changed the exhaust? when you put new piston and rings in, did you bore it any?

even changing the rings and piston can cause it to have more compression. more compression can draw in more fuel, causing it to run rich.  
Keep it RUBBER SIDE DOWN!!

i have a problem with my yfz. it has a full gytr exhaust system with gytr filter and adapter with a k&n filter lid. i recently retimed it and now it cranks over very hard unless i jump it with a car. when it does start the exhaust runs red hot unless the choke is on full. the carb has a 170 main jet and a 45 pilot jet and the mix screw is at 2.5 turns any ideas  
06 YFZ 450- Gytr pipe and header, gytr filter, K&N filter lid

do a plug chop, see what color you are getting at the spark plug.  
Keep it RUBBER SIDE DOWN!!