Pilot flying Piper Aerostar 602P aircraft reported a malfunction with turbocharger system in climb; resulting in a return to departure airport.

Narrative

The flight was scheduled to depart ZZZ at XB:00; under FAA Part 91 for a personal flight to ZZZ1. I was the only person on board the plane. An IFR flight plan was filed for this nonstop flight at FL250 with an estimated time of 2 hours and 11 minutes enroute. Fuel on board at departure was planned at 4 hours and 40 minutes. Weather at both destination and departure airports was VFR conditions and there were no weather hazards forecast for the enroute portion of the flight.The aircraft is parked in my hangar at ZZZ. After the airplane was removed from the hangar; a standard pre-flight inspection was done; and no anomalies were found. I was running ahead of schedule and after start up; I was able to taxi away from my hangar about XA:20. After identifying myself as an IFR departure to ZZZ Ground Control; I was directed to the Runway XX run-up area as is normally the case at ZZZ even when all other departures were using Runway XY. In the run-up area at Runway XX I received my IFR Clearance which was what I had filed for. The first fix in my routing was ZZZ2. After run-up checks for both engines were complete; and everything set for take-off; I called tower to request IFR release/departure. The time was approximately XA:30 at this point. No anomalies were found during the run-up.Tower at ZZZ acknowledged my request for departure and said Departure Control will let him know when they are ready. After 20 minutes approximately of waiting; Tower said they had not heard yet of an expected release time. At the 30 minute mark (approximately) of holding; the Tower advised that a release should be forthcoming. Eventually after almost an hour of waiting and holding for departure Tower suggested I depart VFR and try to pick up my IFR clearance with Departure.During the long one hour wait holding short of Runway XX; I would occasionally 'clear' both engines by doing another brief run-up to un-foul any spark plugs; etc. Both engines appeared to be maintaining normal idle and were smooth running. Tower finally cleared me to depart VFR from Runway XX at XB:30; one hour after calling ready for departure.Tower handed me off to Departure where I received my IFR clearance again. I was cleared to climb to various intermediate altitudes expecting to eventually reach FL250. Over the city area Departure handed me off to [another] Approach [facility] and cleared me to 13;000 feet. Approach handed me off to Center just as I was approaching 13;000 feet. It was then I had a sudden partial loss of power in the left engine. I immediately [advised ATC] and at the same time I [stated] I wanted to cancel IFR and revert to VFR which was granted. Center transferred me back to Approach. At this time; I started to make a turn towards ZZZ3 in case I needed to land as soon as possible. I advised Approach of my intentions. They asked about fuel and souls aboard aircraft. I soon realized that that I could easily maintain altitude and control of the plane and advised Approach that I need some time to troubleshoot the situation as called out for on my emergency check list and that I may not need to land at ZZZ3. I circled for about 3-4 minutes and determined that the problem was the loss of turbocharger boost on the left engine and that the engine had gone normally aspirated. The left engine was operating within normal limits in the normally aspirated mode. I requested a return to my home field; ZZZ which has the best resources to fix the problem; which was granted. Approach and ZZZ Tower were ready for my return which went without incident. I performed a normal landing and taxied back to my hangar. As I descended in altitude the left engine was making more power confirming that it was operating normally aspirated. The Aerostar has a fire detection system which will advise if there is a breach in the exhaust system which can be a more serious matter. No alarms regarding a breach in the exhaust system ever went off which assured me that I could return safely to ZZZ Airport with better troubleshooting resources. I called my A&P/IA as soon as I got back to my hangar. Within a couple of hours; we had the left engine cowling off and we could verify that no obvious or serious issues were present. My local A&P/IA is very knowledgeable about the complex Aerostar systems which is a big help. Additionally; we both consulted with 3 other Aerostar maintenance experts who said the problem with loss of turbo boost is most likely a result of debris (carbon; debris in the oil; etc) that plugged or partially blocked one or several of the small orifices in the turbo controller system. The Aerostar experts suggested we systematically go through each component of the turbo system including turbo boost reference lines; turbocharger itself; turbo actuator; cross over linkages; pillow block bearings; wastegates; linkages to the wastegates; induction tubes to the airboxes.Tests were done on all aspects of the Left Engine with a focus on the components of the turbocharger system. What we found was carbon debris in a restrictor orifice that supplies a reference line to the turbo controller. Additionally; debris was also found in 3 of the fuel injectors to that engine. All carbon debris was cleaned from areas where it wasn't supposed to be including some debris in the turbo actuator. Engine oil and associated filters (total of 4 per engine on the Aerostar) were changed/cleaned. Ground run-ups and tests are showing the left engine is operating normally with turbocharging boost when needed.Comments from the Aerostar mechanic experts universally said the contributing factor for the turbo system getting 'gummed up' was the long hold/idle for nearly an hour where the engine was not designed to idle that long. Additionally; someone noted that on the flight prior to this one; it was a flight in which I did a check-ride in and that the left engine was shut down in flight and re started as part of the exam. One of the mechanics thought that during the re start in the air the engine could have backfired and threw carbon into the injectors or the orifice on the reference line that controls the turbos. Additionally; if the turbos don't properly cool down; they can 'coke up' and get carbon into the system. They said engine shut down airborne on fuel injected/turbocharged engines can be particularly harsh compared to normally aspirated and/or carbureted engines.As an added precaution my A&P/IA; inspected the right engine for carbon deposits and other anomalies by cleaning the fuel injectors; turbo reference lines and checking system integrity. Oil and filters were also changed on the right engine. Both engines only had about 25 hours on the oil. My A&P/IA has now signed off aircraft for return to service.