It sounds as if it might be lean mixture, but we'd need to know more about the symptoms to clarify this, and start working toward the cause.
1. Does it start easily from cold on full choke?
2. How long after a cold start is the earliest you can you move the speed control away from the choke position?
3. Does it run smoothly at all speed settings after warm up, or does it hunt (vary up and down in speed) at some speed settings?
4. How does it handle load (mowing grass) when set at maximum speed?
5. Is the bottom of the fuel tank clean, or is there rubbish in there?
6. Have you tested the spark? (Remove the spark plug, connect it to the lead and clamp it to a metal part of the engine, then pull the starter cord.)
