I ran my M20 for far too long on a very worn carb.
The outcome was a weak mixture that really ran the bike hot.
The carb flange was really badly warped as were the internals.
I bit the bullet and bought a brand new carb.
The bike now starts first kick (not exaggerating).
My extra bit of advice would be to really spend some time on the carb as all your hard work will be undone if it still runs weak.
I am toying with the idea of running the bike without the "bakelite" gasket between the barrel and carb flange. My theory being that the extra gaskets are yet another area for air to get in where it shouldn't.
The bakelite gasket is actually a heat sink, they do work and help to keep your carb body cooler and help stop carb body distortion. I think they are a good idea and they seem not to distort, just measure the thickness of it in a few places to make sure it is flat. You are correct about fitting a new carb, fitting one can transform your bike but it all depends on how worn your original carb is.
My point about the advance lever friction material or disc should be checked to make sure it does not retard on its own when ridding the bike, could be one of the main problems.
As Ian says soild copper head gasket and no sealents at all needed either side of the new annealed head gasket.
