I'm with tx here, start with:
1. Fresh new air filter
2. New plugs, if they are more than a year or two old. - Have a look at the old plugs. I'm assuming your "head" leak is really tappet cover. If you really have a head gasket leak, then you could be pulling oil into one cylinder, and you will have one really carboned up plug.
3. Switch to a Major Brand (Exxon, Shell, Mobil) Premium fuel for a tank or two (they tend to have more detergents), and consider the addition of Techron or similar fuel cleaner. It's worth removing the tank, and fuel petcock and draining anything in the tank to see if you are picking up water and/or sediment. If so, add or replace the in-line fuel filter.
4. Once you're sure the above is all OK, make sure you have the carbs sychronized.
Your comment about increased fuel smell in the exhaust has mean leaning towards the air filter first, but you may be smelling the crancase oil burning (either in the cylinder or on the exhaust).