Help it wont run...

Help...

I've just finished restoring my 1971 ST70 and about 2 weeks ago i fired it up and it started 2nd kick. Ran smoothly although didn't get to run through the gears as it spat the spark plug out. The heads been lightly skimmed and valves re seatted. Barrel honed and new rings fitted.New head gasket prior to it running.
So i removed the head and helicoiled the thread and rebuilt it and tried to start and now it wont start. I checked everything, timing, points, plug and was getting a spark. Stripped and rebuilt the carb. Nothing.
I removed the head and barrel and found 1 of the rings was partially stuck, freed it off , rebuilt again and still nothing.
I then replaced the condenser and points after reading some posts and still nothing firing,
So then 2 days ago engine out, replaced the coil, replaced engine, good spark still nothing even when pushing in gear. It did give 1 small back fire from exhaust. I've put petrol in the combustion chamber and nothing.
When it started previously the lights worked so i'm fairly confident about the magneto. Its a 6V so i know things like earths have to be good. I've double checked the earth on the coil.
The 'O' mark on the cam sprocket lines up with the notch on the head when piston at TDC,' T' mark on magneto lining up with case notch.

Can Anybody tell me where to look next as i'm losing the will to live.... Thanks

Finally sorted this. Was a minute piece of aluminium swarf from tapping the spark plug for the helicoil, stuck to the seat of the inlet valve preventing a good seal.
Started second Kick...runs like a dream!

Hi Brian E, well done, that was a good result after all your hard work