My piston isn't getting gas, spart plug dry.

So I have a 1980 kx250 LTD and my piston isn't getting gas iv checked for leaks iv swapped out the carb all it does is crank. I'm stuck the auto shop is also stumped. I'm really disappointed and need help has anyone experienced or had this problem?

Is it my carb? Maybe the new spark plug,? Any help is appreciated I'm pre sorry if I'm asking a dumb question thanks in advance.

Go through each component in the fuel supply sequence. 
Air hole in the filler cap is clear and allowing air into the tank to replace fuel leaving and not creating a tank vacuum. (you can check this by leaving the filler cap open/off. 
Fuel tap. Does it supply fuel in the open position (with vacuum if required). 
Do you have vacuum to the fuel tap? Look for pipe leaks, kinks etc in the vacuum pipe. 
Are you getting fuel into the float bowls?  Check floats and needle valves are operating correctly. 
If all the above are OK it only leaves blocked jets and / or major air leak on the carb to head mount rubbers. 

This is all assuming you've not had the top end of the engine apart since it last ran. 

If still no joy take it to a old skool bike shop with proper mechanics not computer jockey fitters. 
 

Try to spray some break cleaner into the air box or carb inlet, If you have the correct cam/ignition timing, spark and compression, your engine should start.