250-500cc New project

What he said and make sure your wash bung isn't in the pipe Is your plug wet after kicking it a bunch?
I would kick it with choke on and full throttle, maybe at least get some back firing, plug should be wet then if it doesn't start. If timing was off also should cause back firing. Probably that carb, post up which jets are in it and maybe someone can help.

ps, could haul it to a long downhill....

Fuel getting in there? Close the petcock, open the fuel bowl screw, turn open the petcock and allow the fuel to run to confirm that fuel is entering the system.
Once that is established, and leaving the fuel system open, tilt the bike over to around 45 degrees slow till it's flat on it's side, if the fuel starts to piss out of the vent lines before and on 45 degree, the needle is stuck which can make starting a PITA cold.
If the timing is correct and you still can't start the thing, maybe get a a tow and bump start it in 3rd or 4th gear?

Tried everything just get the occasional backfire, when bike is laid over fuel comes out vents when bike is almost flat. When we tried towing it it would fire for a few seconds at a time then die. Suspect weak spark is the issue was hoping not. Still have the original motoplat ignition in it that are known to give weak and inconsistent spark especially if not used for a long time. Have double checked jetting specs and settings, it should run. Now have ordered a PVL ignition. If this bike was free it still would have been too expensive!

we tried warning you! lol..too late now, but will be nice once set up..

And you've not got a suppressed plug cap with a suppressed plug?!? Those give weak sparks when there breaking down the easy way to tell is to crack of your multimeter and get an ohm reading on the bad boy, anything under 4.9k is fine even a hair over and the resistors breaking down an that's not gunna help matters.

It was the ignition at fault all along. Installed a HPI ignition, bike starts first kick and runs like a dream.