Hi Joe, I realize this won't help you with your current problem, but I'll throw this out there since it seems you repair, or are starting to repair, a pretty wide variety of machines. You may want to look into taking Ray White's sewing machine repair class if one is offered near you:
http://www.whitesewingcenter.com/repairclass.php
I've sung the class' praises on other forums as well. As you get into more and different kinds of machines, you'll find it's impossible to find Adjuster's Manuals for all of them, or even worse, sometimes you go to the trouble of tracking down the manual--and the part you need to fix isn't even mentioned! That's when I've found Ray's training handy, as he teaches you to "learn" a machine by looking at the parts. It helped me fix some problems on a Singer 111w155 industrial that I couldn't find any assistance for.
As for a more short-term solution, one idea might be to do a little research and find out if there are other models that are similar to the 413 and 538, in the way that the 403 and 500a have a few things in common. Then, if you can acquire an Adjuster's Manual for the similar model, there might be enough overlap in it to help you.
The other short-term solution is to jump on the Vintage Singers Yahoo Group and do a search of the archives to see if anyone's had, and fixed, the specific problems you're having. Failing that you can post a query in hopes that someone's got the fix.