I got my MC21 running again after sitting for a few months. Started and rode fine. Then I took off the gas tank and seat cowl to do some electrical work and change the plugs, left it overnight as a massive typhoon hit. Now it roughly idles and stalls if I give it any gas. Using the choke raises the revs a bit but as soon as I give it a bit of gas it dies. Any ideas? Much appreciated.

Well here is one theory:
The low pressure of the typhoon system caused the pressure in the fuel tank to drop.
Perhaps a faulty fuel tank breather only allows air to escape the tank, and not re-enter.
So after the typhoon, the pressure in the tank remained low and this is preventing fuel from running into the carb float bowls.
OK, that is a bit of a long shot,... ,.. but might have a slim chance of being true?

It sounds like a lean condition to me.
Either there is an air leak downstream of the carbs or fuel is not getting into the float bowl rapidly enough.
Check that fuel flows freely out of the fuel tap and into the carbs.
Check the inlet manifold rubbers for leaks and that the boost bottle 'thing' is plugged into the rubbers properly.
Check the downstream hose clamps are sealing the carbs well.

If none of that works then maybe check the float levels and give the carbs a good clean out.
Maybe if there is an inline fuel filter, check it is not restricting fuel flow.
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...

I forgot to say that if it is a tank pressure related problem, then just opening the fuel cap may provide a temporary fix?
Things are never that simple though.
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...

Thanks for the input TuckerBag. I was able to get the bike running after checking the fuel tank and DRAINING the floats. Runs well after that, must have been some contaminated fuel I am guessing. Cheers!