You're right, if the bike is stock make sure everything in the carbs is stock, clean them up and check the RC valves are set correctly.
While the carbs are off have a peer inside and make sure the reeds are OK.

Then if it doesn't work on clean plugs something is actually wrong.

Check the plug caps are well attached to the leads. I chased a random spluttery missfire for a couple of weeks before the cap and lead actually parted company.
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