There is a product called Oil Eater that can be purchased at various locations, but I get mine at Pep Boys. It comes in a concentrated spray bottle. You can dilute with water to a 50/50 mix and ue it on your bike's engine, wheels, etc. YOUR ENGINE MUST BE COLD, or damage can occur. If used on a cold bike, this is THE BEST stuff to clean all exterior parts of a bike. It's quite an amazing product. Hope this helps.
I've also used WD-40 to remove tar, etc with fantastic results.