It may be worth checking the carburettor bakelite insulator, they do tend to crack. Other things to check are the gas safety devices (hollow bolts) located on the cylinder head, check there is no gas escaping from these bolts if so this indicates a blown head gasket.
Have you checked the contact breaker points and then the ignition timing? which should have been the first check.
And like other are saying also check the valve clearances.
