One of the local hotels here in town got a new to them but clearly worn the hell out 2012 JCB 409 wheel loader with the air cooled diesel engine for snow removal. After my boss hauled it up here from the dealer they bought it from in Idaho Falls, it wouldn't idle or hardly run at all. They sought me out to see what the heck was up with it. I did simple maintenance on it that should have been done at the dealer, filters and oil change, and it now runs and will idle. When it is cold, it runs fantastic and will go full speed and do loader things. However, when it gets some heat in it like all engines naturally do, there is a lot of black smoke and it will no longer do loader work. I blew out the primary filter and the secondary filter was in almost new condition. Being as it is an oil/air cooled engine, is there a temp sensor somewhere that helps control the fuel mixture? The dealer put a new cluster in it so we have no idea how many hours are on the machine. Judging by the paint wear and such on the inside, I would guess it has 2000+ hours.
Check air filters
I pulled both air filters and ran it a round the parking lot of the hotel. No change. We have about 7 feet of snow on the ground so dust wasn't an issue. The problem persisted even after the removal of the filters.