Remove the inside of the pullstart and clean it out. Sand is a killer no matter what the item is.
Here is a post for help on it:
http://www.hpibajaforum.com/forum/showthread.php?t=3694
I found it to be extremely easy,
First make sure that the housing is not broken, the housing being the outer part of the assembly.
It should have a place on the outside of the ring that the end of the spring should slip into; the end of the spring should have a bend in it that will slip into that slot.
The other end of the spring (the end that will be in the center) should also have a bend in it, look on the back of the black plastic center piece and there should be a corresponding place for the spring to hook onto.
slip the outside end of the spring into the slot on the outer part so the spring can be wrapped counter clockwise, and wrap it in.
now if (and it should be, or you can bend it slightly) the hook on the inside end of the spring is about 1/4 inch from the center point it will automatically slip into the black plastic piece when you put it back on.
Try a test fit, put the black piece back on and turn it counter clockwise......it should grab the spring and build tension....if it does great!!! if not you will have to mess with the spring a bit to get it in the correct spot to grab it correctly.
Your almost done
first release all the tension and remove the black piece.
With the string placed through the hole and both ends connected,(one in the handle and one end knotted in the center black piece) wrap about 2/3 of the string onto the center black piece in a counter clockwise direction.
Now put it in place and screw the assembly back together.
There is a notch on the side of the black piece, hold the string there and begin wrapping counter clockwise approximately 4-5 turns.
now SLOWLY let the spring tension pull the remaining 1/3 of string in.
If every thing worked right you should have a rebuilt spring, if not go to a hardware store and they can show you how to do it.
One thing to check that it extremely important....when you pull the cord to it full length the knotted end should stop its extension NOT the spring. If the spring is stopping its extension then it's wrapped to tight, unwrap one coil of string on the black piece.