It could be the plugs, might have flooded them beyond the point of return when it was laid over (would have been idling on its side while lots of fuel was possible let through). Only way you'll know for sure is to try a set of known good plugs in it. Sounds unlikely but bad plugs will cause a lot of head scratching, especially seeing as you've taken them all out and possibly put them back in other cylinders.

Check all the major electrical connections also: battery terminals, negative leads, starter relay positive connections, coil wiring etc. See if any wires are loose on these parts.

was it doing this befor you did the carbs?
are all carbs getting fuel ?

what did you find in the air box/carbs?

GOOD NEWS

Took out the HT leads sprayed some wd40 up then the cleaned then out, placed a old plug in the end and checked they were sparking correctly. checked the spark plug gap, replaced all the plugs/leads.

she fired up first go and is running beautifully! very pleased, now waiting for the snow to go away!

how many times has it been some thing so simple !!! a reminder to us all. lol

glad you got it fixed