PA32 pilot reported the engine was running rough during initial climb. Pilot returned to departure airport.

Narrative

I planned to fly from ZZZ1 to ZZZ2 on Day 0 with a halfway stop at ZZZ. Departed ZZZ1 at XA:45. Landed at ZZZ at XD:15. Departed ZZZ at XE:26.On climbout for the ZZZ to ZZZ2 flight; the engine started to run rough. I notified ATC that I was returning to the airport. As I reduced the manifold pressure to start the descent; the roughness increased and the engine monitor showed cylinder 3 EGT & CHT dropped off the scale. At this time; I requested priority handling and ATC cleared me straight in for runway XX at ZZZ.I landed normally and taxied to the FBO. Explained my problem and two technicians helped me to troubleshoot. They conducted a thorough inspection. It was during the trouble-shooting that I discovered the open primer control. One of the technicians is a pilot as well and we conducted a very thorough run-up; and the engine operated normally. We both agreed that the roughness was the result of the open primer.I provided my information and summary of the events to two gentlemen from airport operations.At this time; it was late in the day; and I was too tired to continue fly safely. My wife and I decided to adjust our plans and not continue to ZZZ2. We returned to ZZZ1 on Day 3 around XA00 am. The return flight was normal.Corrective action: Double-check checklist items at all times.