Crf250r Wont Start After Full Rebuild

hey everyone,
man has it been a long time since ive posted here..
Anyways, I blew the big end crank bearing in my 2006 crf250r. it has sat for a while but i've finally got it back together. (new crank, piston, rings)
I've been trying to get it to start with no luck. every 20 kicks i get a pop or a backfire, and on occasion like its going to start to run but it stops. I know the motor is assembled correctly.
No matter how many times i kick it will not start.
I've checked the valve clearances (original head and valves but shimmed with in spec) , adjusted the cam timing because i had thought it was a tooth off. Checked the electrical, it has spark (im going to get a new plug though), tried unhooking the kill switch, cleaned the carb including pulling out and soaking the all the jets in a cleaner. it has a clean airfilter.

This is driving me crazy. i dont know what else it could be. If anyone has ideas please send them my way.

thanks

Thanks for the replies,

Well after poking around a little more i have narrowed it down to an electrical problem. It had weak spark when i first tryed to start it (i assumed the plug was getting pooched). Got a new plug today. the bike now has no spark at all. checked all the wires with a ohm meter for continuity. everything checks out. all resistance values in spec. i pulled the magneto side cover yesterday, and found some small metal shavings on the pick up coil. Could this be the culprit? i had cleaned it off as best i could, but i may have picked up more. I changed the engine oil because i had to take off the cover.

Im going to pick up a magnetic drain plug, and I am going to change the filter and oil as soon as i get it running.

But could the shavings on the pick up coil be the problem (no spark, lost the small amount of spark it had after i had re assembled it).

Thanks again,