L4-134 At first start-up idling when cold, valve/tappet is sticking. Runs on 3 cylinders for about 2-3 minutes. Sounds like popcorn machine, rough, backfiring, etc. Suddenly clears up and engine from then on runs fine. Does not stick when warm, only on initial cold start up. Engine had been sitting for about 10+ years prior to recent startups. First three startups, had no issues. Ran fine. It's only the last 3 startups that the problem has arisen. Add an oil additive, fuel additive, or both? Take it out and drive it ? (splash oil all around) Fuel and engine oil all fresh, recently replaced. Thoughts?