A problem with carburetion is mostly due to bad mixture setting. Either the engine will not breath properly upon acceleration (engine running lean) or give black smoke from the exhaust (engine running rich). Following factors also contribute to bad fuel economy:
- Fouled spark plugs and/or Incorrect plug gap
- Bad point, condenser, rotor, distributor cap
- Incorrect point breaker gap
- Incorrect ignition timing
- Incorrect valve clearance.
- Clogged air cleaner
- Loss of compression
- Carburetor jetting issue.
These are common issues related to high fuel consumption. Others can add if i'm missing something.