2006 CRF250r Starting Issues

A couple months ago I gave my bike a service (engine oil, transmission oil, spark plug, cleaned air filter, bled the brakes, cleaned out the carby and replaced seals) and went away for the weekend.
First day on the track it ran as good as new, but took it out the second day to a mates farm and rode for about 2 hours. Came back to the house, let the bike sit for about 20 minutes, got back on and started it up but remembered I had to do somethin so i turned it off...and it wouldn't start after that.
Long story short, It needed a rebuild anyway so while I had it apart i decided to do Piston and Rings and while I had the head off I figured I'd check out the valves, turns out they were pretty well gone.
So I've been saving for weeks to get all the parts (Piston and rings, all new valves, gasket kit and new chain and sprockets as well)
Got it all back together and gave it a few kicks, after a while it would start for half a second to a second and then just cut out. So I checked the spark plug, turns out the spark was pretty weak, so I had to wait a couple weeks cause my mate had my spare one that was working good. So i tried starting it on the weekend with the working spark plug and nothing...I have a good spark, I have compression, its getting fuel, but its just not starting!
Any ideas anyone?  

What about the timing? Did you check to see if you lined it up right when you put it back together?  
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I love this dirt stuff.

Yeah mate, that's the first thing i checked...
I don't think its anything i put back together wrong, cause it wasn't starting before i rebuilt it
It's got a good spark, getting fuel, got compression...so theoretically it should start, right?  

I have had kill switches go bad and cause these symptoms, maybe check that next.  
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I love this dirt stuff.

If it was the killswitch, then it wouldn't spark when i pull the plug out and earth it out and kick it over...right? So I dont think its the killswitch...any other ideas anyone?  

Its the weekend of the 4th and alot of people are out and about 2019. Give it a little time and someone might have some ideas as to what it might be.

Will keep bumping to the top till the guys get back from the weekend.

Valve clearance?  
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I love this dirt stuff.

Thanks mate, nah all new valves...and set all the clearances
That's what i thought it was at first when i stripped the motor down, cause the clearances were way off...  

No ones got any ideas...?  

not already mentioned  
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Brand new yammi 450 =  $7399
To see a honda burry it at the track=PRICLESS!!

Is it a good dark blue spark?  
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I love this dirt stuff.

mx_rider2019:
No ones got any ideas...?

Well, timing was the first thought but you have verified that.  Does it try to start at all (any popping/backfireing etc)

How old is the fuel in the bike/carb?  
.
Dave H 2006 YZ250
(2008 YZ 125 son)

With the first spark plug, it was a weak one. But when I swapped them over it was a good dark blue one...
well when i first tried starting it after i rebuilt it, it was backfiring for a while then like i said, it would start for half a second - a second and then just cut out.
and i left it for a couple of weeks and then tried starting it and now nothing...  

try pouring a tea spoon of gas in the cylinder then put the spark plug back in and give her a try.  
My ride -2002 KX 125