I've had very good results using dishwashing powder although I haven't tried dishwashing liquid yet. I dissolve about a spoonful of powder in boiling water then let it cool until it's just barely comfortable to dip a finger in. The engine in the photo only needed to be soaked for maybe 10 minutes but the muffler took quite a bit longer because it had badly baked on castor. The piston only needed a few seconds before all the varnish rubbed off using my fingers (that's after about 30 hours of running in a 4 stroke). Because it's designed for washing dishes it's obviously not poisonous