Loco Cleaner(Locomotive) CUTS through the THICKEST OLDEST gunk,
but not sure if its available through Napa or car stores, (...get it through my dads shop)
I would think its available to general public but i may be wrong... dilute it with water, o_0 3/4Gal. water 1/4Gal. Loco(<that still may be really strong but its what I mix...)
But I use it to clean my chain, before adding more wax, and the rear sprocket where that layer of gunk builds up below the chain, little on a paper towel, wipes clean with minimal effort, and the part where the exhaust connects to the muffler, the air box gets a NASTY film of oil the collects dirt, and just builds rather fast, and it all comes off the same way its a great soap!! gets stuff clean, but I'm stingy with it, and use it for basically those two things, should scrap that iron dirt right off, The Usual Spray, Soak 5 mins. then wipe, scrub, or what you will with what you have,