Many times starting problems can be traced to spark plugs with high resistance. The high resistance of the plug can make it very difficult for the magneto to fire a spark, especially at lower starting rpm. Long-term wear of the magneto points can become a problem as well. The plugs should be checked at the annual condition inspection, every 100 hours, or whenever starting problems are noted. Tempest makes a really nice go/no-go tester, but you can use a normal ohmmeter as well. Resistance should be less than 2K ohms when tested across the center electrode. Most new plugs will indicate around 800-1200 ohms.