I agree with using the baking soda method of Tartan. You really, really do not want to try to wash an old quilt top. Aside from dye bleeds, there is a huge risk of distortion from the fabrics shrinking at different rates. The distortion from shrinking issue is removed if you are washing a quilt because the quilting ties the quilt top to the batting, and the batting prevents distortion from uneven shrinkage rates.
Also, leaving the quilt top outside in the fresh air for a couple of days can work wonders. If you do this, be sure to leave the quilt in the shade (light fades fabric dyes fast). If it is not in a protected area, such as a screened porch, layer it between sheets in the shade (to protect against droppings from birds and other critters) and anchor the edges securely with rocks or bricks.
But, I think the baking soda will work.
If you do have to resort to water, I highly recommend basting the quilt top to an old white sheet before immersing in water. The basting lines would need to be pretty close together, similar to quilting lines. The sheet would protect against fraying of the seams and also will help somewhat with uneven shrinkage provided you lay the quilt top/sheet combo out on carpeting to dry and block it as it dries.