When this first happened I shut the engine down.
Then started to check the areas you are mentioning. The air filter (2 years old) is going to be replaced along with the elbow now. It is intact so I ruled that out.
Fuel and filters (replace last year) were clear and showed no debris on the filters. As part of my rounding maintenance they get replaced every two years anyway. Cheap insurance.
I removed the impellor to inspect that (new in April). I replace that yearly anyways. It was fine. The alternator spins freely and no telltale noises coming from that either. The engine ran as usual, just up to 1200 rpm as I mentioned so....
I really want it to be the Elbow, given its age in service, but I am prepared, as best anyone can be for the alternatives.
I'm saving the injectors and their testing, as a last resort. If I have to get involved in them I will, but hoping for the elbow and the very obivious signs I see from just the initial inspection I've done.
Today I just contacted Yanmar and Mack Boring (here locally) for the correct part numbers. I've gotten numerous part numbers all weekend long from the internet and burned myself out looking. These parts numbers change like the weather.
I've got a current list and working with a local Yanmar Dealer to get them as soon as possible.
Other than that, it's where I am currently. And hoping for the best as I go along.