Problem with duplicateMovieClip

first have a look at here:
http://www.zone.ee/avp2estclan/teksti%20katse4.swf (click 100* then it plays faster)

Wht i wish to do is tht the black ball(auto) leaves line behind, first im trying to just make it leave , duplicated mc behind. But i fail. Anyideas how to make it work.

Here’s the current code:
[AS]stop();
loadVariablesNum(“replay.php”, 0);
kiirus = 250;
arv = 0;
function koordinaat(a, b, c) {
return ((a-b)/c);

}

function uuenda() {

ax1 = koordinaat(bx1, 370000, 496);
ay1 = koordinaat(by1, 6376000, 488);
arv = arv+1;
bx1 = eval("x"+arv);
by1 = eval("y"+arv);
setProperty(auto, _x, ax1);
setProperty(auto, _y, ay1);
_root.auto.duplicateMovieClip("auto2", 1);
    auto2._x = ax1// puzzeled
    auto2._y = ay1//puzzeled as well

}[/AS]