The manual I have been reading lately for the Hobbit states these things as issues for rough idle. Since you figure the fuel system is OK I'll skip to the electrical and other issues.
- Check your spark plug and/or buy a new one. Properly gap a new plug.
- Check you points for dirt/corrosion and make sure it's gapped correctly.
- They do mention a blown head gasket as a possible cause. You should consider re torquing your head to proper specs since engines expand and contract with use.
- Choked silencer on your exhaust.
The case gaskets if worn usually only cause air leaks which makes for a lean condition since more air is drawn into the case. Since you mentioned your spark plug was dark that would indicate it's not running too lean and probably your issue is with the ignition system. My guess would be the contact breaker points or old plug. I don't know much about the coil but I imagine they do fail eventually that could also be an issue. Hope this helps.