1. Do a compression test.
2. Do a vacuum test.
3. Do a fuel pump pressure test.
Then, re-check timing under varying speed paying particular attention to mark movement. Ensure the distributor cam lobes are not unevenly worn, vacuum advance functions properly, the points rubbing block is intact, and the point assembly is securely fastened. Next, check the air filter and inlet for obstruction. A sloppy choke plate is suspect too.
A broken valve spring, as well as a sticky valve will cause the problem you've described; improper valve stem adjustment, too. But, without 1&2&3 done first it's hunting and pecking. Harvester is right on the money about plugs - they are the 'eyes' of an engine and reveal much about its efficient use of fuel. Happy hunting.
