I performed a wash on a H&S STAD 250 engined start cart, by necessity as the thing couldn't maintain air delivery before the EGT went off-scale high.
We used a pressure-washer with soap dispenser filled with ZOK 27 compressor wash fluid.
Cleaning while motoring didn't seem to accomplish much after a further test, so we started and idled it and then tried again while running. It required one person to lean on the relight button but we successfully emptied the tank though it.
Testing under load showed we now had a 50°C margin instead of being over limits, perfect.
This was a very tired engine that had possibly never been washed before in its life but it had a remarkable result.