Ethylene glycol, aka anti-freeze. Most people use undiluted but 50/50 pre-mixed will work but take a little longer.

After breaking in two engines my 053 pipe was pretty gunked up from running it rich that long. So if you're pipes have some miles on them it might not be a bad idea.

The most common method is to throw them in an old crock pot and let them sit overnight or longer depending how dirty they are. I put mine out in the garage so the fumes don't bother anyone. I'd also recommend using a respirator as the fumes aren't anything you want to breathe. Sounds crazy, but it works lol