I have found that most of your citrus cleaners work well on tar but you have to be very careful not to get it on your aluminum parts that have clear coat because it will damage clear coat.
I use this stuff from work called MKG Orange Solv II. Works good and smells good too.
Try plain old ciggarette lighter fluid like momma used to make, I think you'll find it to be the miracle formula you seek. Also this will get the old black nasty oil off your engine if you have any.
Hey ...i'm not real sure about tar but i had a instance when I was riding this past summer and a darn plastic bag got wrapped around my pipes....Dang ! what a mess...I tried everything...rubbing stuff of all names ...non worked at all...Tried tar remover...lighter fluid...Bug and Tar remover....I was thinking ..man I gotta get a new pipe coz trust me it was a mess...So my wife says to me ...Why don't you use oven cleaner...I laughed at her ...but was willing to try anything...so she went to the store and brought me back some Easy Off...over cleaner...was for cool ovens..I think the other has lime ...anyway I applied it per instructions like in a cold oven...and Guess what ..it worked and did not harm the chrome at all. In fact the pipes looks good as new...so when all else fails ask your wife are your Mom...
Just plain fuel does the trick for me. And if you don't like the smell of that you can use lighter fuel.
T-Cut paint restorer works also but be sure not to "rub" too much.