This may have been suggested, but try running it under max load with fresh fuel for several hours. Partial load doesn't help. Clean or replace the air filter even if you think it doesn't need it.
Use a IR pyrometer to check the temps of the exhaust ports. They should be very close, the one closest to the water pump is usually the coolest. If one cylinder is cooler than the rest, there's an issue with that cylinder.
As suggested, while the genset is running under load, crack an injector line, one cylinder at a time. Watch for several minutes (yes, messy) and observe if smoke diminishes. If so you have a leaky injector or low compression on the cylinder.
You can check compression with a tester easily. You can swap injectors to see if the temp difference moves.
With any luck it's just a mouse nest in the air filter.
How many years did it sit without running?
