I had a muffler that was gunked up pretty good. I put the muffler in zip lock bag with about 1/4 cup of househould amonia and let it set overnight or longer, then just washed it off with dish soup and a brush. It came out looking pretty good. It doesn't have to sit in the amonia just be enclosed with the amonia vapors.
I used this alot in the military when moving out of housing and had to clean the oven racks and drip pans on burners and it saved me alot of elbow grease not to mention many brillo pads. I figured since the fuel uses alot of mineral oil for lubication that this was the same all the grease used on a stove. I not sure what the amonia will do to the internal components of the engine, probably nothing.
Hope this helps.