Constabulary

Wiz, I think the bobbin thread belongs there. Thats what the gap in the bobbin retainer is for. Other machines have it this way or very similar too

I have not worked on machines with this type of hook for a loooong time. I guess its a thread take up lever or hook timing issue (or both).

I may have complete tension unit.... will check this weekend. But I´m in Germany.

Don´t know what N66 is but reg. needles - 135x17 and 135x16 have the same dimensions. 135x16 is a needle with a cutting tip for leather whereas 135x17 is for fabric but you can sew leather with it as well. The cutting tip depends on what stitch appearance you want to achieve. This brochure (direct download link) may give you more information reg. needle tips: https://www.groz-beckert.com/mm/media/en/web/pdf/Sewing_machine_needles_for_leather_and_technical_textiles.pdf

when I make turns my needle tip is still in the material the way that I see needle scarf - maybe worth giving it a try. BTW - have you opened your slide plate / hook cover and watched the thread when it slides around the hook? Maybe it keeps hanging somewhere maybe on the bobbin case opener or the hook needs some adjustment - just a guess.

Depends on how heavy you want to sew. Maybe the 45K1 can replace the Singer 7-10 beast and the 78-1 I guess is a good replacement of the Tiny 15K. Thats at least what I would choose.

No you cant but you can move the needle holder a bit sideways. Loosen the screw above the needle clamping screw and try to move the needle holder. It may be stuck if so try to remove it and clean it from oil, gunk and dirt and try again. Whe moving the needle holder make sure the tip of the shuttle hook does not touch the needle.

It was designed as garment / fabric sewing machine but if for sure can handle some light garment leather. But since the seller is your neighbor why don´t you go and test it with the materials you want to sew. Buy some leather needles if he has none and see what it does. Maybe you have to swap out the motor and buy a new servo motor this swill give you a better speed control.

I do not know the machine in particular but to me t looks like a drop feed + needle feed machine with roller foot. I´m not sure but it seems to be a short stitch machine (like 2-3mm or so) cause the slot in the needle plate is quite short. I think it is designed to sew shoe uppers and so forth. Not sure if it suits you needs but I would not pay 1100 for it.

Singer 29K1 or 29K4 manuals will work for the 29K3 as well. There is not a major difference. Maybe a side wheel or a wax pot but nothing really important when is comes to threading or sewing.

Please post pictures of your machine from the underside. I have a Singer 132K6 which is the predecessor of the SK-6. On my 132K6 there is a bushing for the hook drive shaft that is held in place by a set screw. I think when you loosen the set screw you can try to move (hammer) the bushing carefully to the right a little bit so you can move the hook a bit to the right as well. The set screw is probably covered by the bobbin case retainer assembly so you have to remove this first. If there is an adjustment ring on the right side of the bushing loosen its set screw first. I don´t know your machine but that's what I would try.

Its down for quite a while. I have no contact data, sorry. I guess he is out of bushiness or maybe even passed away - I don´t know.

I seriously doubt that there is a major difference between any of the Juki 341 clones. They are all made in China and I doubt there are as many factories that makes them as there are "brand names" that sells them. From what I read Atlas Sewing Machine Co. has a very good reputation. IMO it all depends on how well the machines are set up and how well they are supported by the brand name owner / dealer. I doubt that any of the 341 clones will wear out sooner or last longer than others. If you are new to sewing machines I would buy from a dealer with a good reputation and that is close to you just in case you have troubles that you cannot solve by your self. Sending sewing machines back for service is always a risk so you either service them by your self or you better take it to the dealer by your self. Packing up sewing machines properly is quite a task. Do not expect they will be handled carefully so packaging is the key and shipping is expensive! Since you are in Denver why don´t you check with Ralph´s Power Sewing, they are in Denver! https://powersew.com/ I´m sure they have 341´s too with just a different mane on it. Maybe even used original Juki 341´s.

I once owned a 7-10 IIRC I have used 794 needles. However - have you tried them? Even if it is not set up for 794 needles you can adjust the needle bar height to make them work. thats a tiny domestic machine I´m sure nothing from it will work on your 7-10 beast

I´ll respond to your PM later - anyway the website for the Adler Parts is down for quite a while already. This is the dead link: http://altenaehmaschine.de/ I deleted it from my bookmarks a few month ago.