I'm not sure what gunk you are talking about but the burnt on plasti sounds feasible.
Me, I'm still learning the art of using the most appropriate cleaner for the job at hand. What I've been doing lately though is Initially wiping my bike down ie wheels, any oil flung off my chain and mostly anything else, down using kero.
Gets all the dirtiest dirt (is there clean dirt stacka lol?) completely off before I use my soapy sponge. Works well for me but a good supply of rags are essentia. Also stops the greasiest grease (I'm on a roll) going down our waterways.
