PA-28 Cherokee/Archer/Dakota/Pillan/Warrior
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Flight Instructor; Flight Crew Commercial
Flight Crew Last 90 Days 226; Flight Crew Total 785; Flight Crew Type 350
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Diverted; General Flight Cancelled / Delayed
Aircraft
I was flying from ZZZ to ZZZ1 via [airway] at [altitude].  I had just leveled out at cruise altitude when the engine began to run rough. This part of the route was most treacherous in terms of landing options being in the mountains. I chose to turn from my position direct back to the ZZZ2 airport and towards [land mark] which I thought would be a better landing spot to put it down if the engine did decide to quit. I maintained altitude to stay high while enroute back to ZZZ2 to keep our options open. The power did not fluctuate it was just running rough. I did go through the checklist to which nothing seemed to change the roughness. I held off on the magneto check because I wanted to get to a spot; I was comfortable with losing power to do the mag check (over ZZZ2) I listened to the ATIS and was going to begin my call up to the Tower from 15 miles to the north.  At this point before I made the call the engine began running extremely rough. This prompted me to request priority handling with Tower.  Tower gave me the landing clearance then and there for Runway XX.  We communicated and I decided to stay high until over the top of the field then spiral down for the airport to stay within gliding distance. I did this because I know that historically most engine problems just seem to get worse when you make changes to the throttle and since I had to reduce power at some point to land the plane; I decided I was going to stay in a position to land safely with no power if it did happen. I spiraled down to join a left downwind for Runway XX and landed and taxied in. On the ground shortly after we performed a magneto check in which the L magneto dropped approximately 800 RPM and the engine began running extremely rough. The pre-takeoff magneto check was performed prior to the flight with no issues and magneto drops within tolerances.  The airplane is in maintenance right now and I have just heard today that there was apparently a plastic piece on the magneto that should've been removed when they were installed that came loose and somehow affected the timing of the left magneto. In terms of the human factor; I had my girlfriend who is an instrument rated private pilot in the left seat; and her mother in the back. Geographically I didn't want to take any chances so we turned back towards the [landmark]; airport with a long runway; and flatter terrain because I have never dealt with an engine running that rough and couldn't figure out why and expected the engine to fail. I believe my only inaction was checking the magnetos which I purposefully refrained from until we were over the airport. By the time we were over the airport and had the field made; I just decided we would focus on landing and deal with it on the ground.
PA28A pilot reported an uneventful diversion due to critical engine malfunction.