Too many twists in your string can weaken it and cause it to "fuzz up" easily because more stress is added to the fibers of the string material. You do need some twists in the string to keep the serving tight. Hopefully, all the servings are going the same way and I just twist the string the opposite way the serving is wound to keep the servings tight. For starters I like to get the string length close with about 15 to 20 twists (for a single cam string) and then do the end and center servings under about 200 pounds of tension (all I can apply with my set up).
If you can work with wood, you can make your own jig for very little money. A string serving tool is only about $14.00 , use .018 inch serving for the loops and end servings and .021 for center serving. A 1/4 pound spool of string material will cost about $35 to $40. You can make a good number of strings and cables with a spool of material and two spools of serving