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The feed dog cam is way off/ out of time. This cam is what is being hit by the stitch length slide.
Feed dog timing should be adjusted so that the top of the feed dog teeth are level with the needle plate when the eye of the needle descends into the hole of the plate. The feed timing adjustment on your machine is a cam on the main upper shaft.
To adjust the feed timing set the needle eye so that it is just going into the needle plate hole. Then loosen the set screw slightly and turn the cam to have the top of the teeth just go below the level of the plate. Tighten the screw and test.Note that when you advance or retard the feed timing you will also increase or reduce the maximum stitch length. The picture below is of a different machine, but very similar and I have marked with a yellow arrow the set screw that you need to loosen. Set the stitch length during this adjustment at midway so as to get the slide out of the way of the cam.
Thanks for the diagnosis and what to adjust.
When I worked on this before I tried to move that cam. I couldn't get it to budge. Is this the only screw for that cam? If I remember right (questionable at 12:15AM) there are two screws. When I had them broke loose the last time I didn't know what to use as a reference point. Thanks, I do now. What is the best way to turn that cam? Will try this tomorrow and get back with you.
Here is a link to a parts manual with pictures of the 99K:
Go to page 7 plate 2394.
The oscillating hook part #32520 is the big round thing
Towards the bottom of the page, part # 32594 is the part that attaches the bottom of the hook to the levers that make it work.
Page 4 plate 895.
The feed dog is the toothy thing, part #32600. It's screw is on plate #2400 and it's part #191C
The part above it, #33635, and the two parts to the right of it, #'s 32529 and 33613 are part of the thread take-up lever assembly.
I haven't found where the screws for that are located.
So to review, I need the oscillating hook, the feed dog and screw, and the thread take up lever assembly.
Note one of the screws on the thread take up assembly has left handed threads. It's the one with the concave slot. Easy to see.
If this doesn't help, PM me your email addy and I'll take some genuine digital images and send them to you.
Just make sure when you get the basement you get:
( In order of importance)
1 - A dry basement, check carefully for mold, mildew, or previous flooding.
2 - Ample head room for the tallest person in your family. I about killed myself in the basement in the first house we lived in here in Springfield. Those floor beams are solid.
2a - Wide tall stairs down to the basement. Having to bend over to avoid a concussion or bend and twist to get down the stairs is a pain in the a ..... 'em butt.
3 - Ample windows that open for ventilation.
4 - Light, you need lots of light. That can be worked on though.
Hope you get pretty legs a good place to set up.