Constabulary

you have to pull out the foot bar. You have to loosen / remove theses 3 screws too - see rec circel. Alos make sure you removed the return spring that pulls back the presser foot (not meaning foot pressure spring)

to me it looks like a Juki 555 fabric / tailoring machine of some kind

you probaly have chosen a short stitch lenght or the bell crank lever is worn down (or both). Maybe teeth on the presser foot are dull / worn down. Also theck if the "gib" on the stitch lenght adjuster is installed correctly. See pictures here:

Hope this helps 😉

Looking at this picture I belive the Singer 107w hinges are the same. https://www.college-sewing.co.uk/202258-210844-machine-hinge-hook-singer-107w.html?srsltid=AfmBOopJb1ilppAZoElRixRJKheoNF9ftFELvIPI6Cw_MaVvn7w75taq https://armastore.eu/210844-singer-107w-hinge-hook You will get them on your end as well just check the numbers online But I would probably buy the female part too - you never know....

Looks like the shuttle race is not installed correctly,

in addition to my last post: Just in case you think the 29K71 (or other patcher machine - no matter the brand) is a great heavy duty leather sewing machine ... it clearly is NOT! Its is a machine for repair work but not for producing leather goods. It may work for odds and ends in your privat environment but not for pro leather work or for producing goods you probably want so sell. I´m barely using mine but if you need it for "odd work" then it is indispemnseable - it can do work that other machines can´t do.

I have sewn close to 12mm webbing with mine. But thats not what it should sew every day. I just played with some parts setting, no other chnages no grinding or welding or so. And stitches become shorter the thicker the material is.

A Singer 144w as it seems. More or less the "fat brother" of the Siger 111w.

depends on what leather product you want to sew with what needle / thread size. By design it is a high speed tailoring machine and not really a leather sewing machine. It would not be my 1st or 2nd or 3rd... choise.

I´d check the needle - hook timing / needle bar height. My guess is the NB is set a little to high.

I´d try this by hand cranking the machine but again the needle hole has to be wide enough for 2 thread strands + needle. 214x1 / 328 needles have the same dimensions as 216x1 and I´m using the 214x1 in my machine w/o problems. 216x1 has a slightly different needle tip they have some kind of channel in the tip - not sure why. In case the hook tip hits the needle you have to move the hook a bit to the right and probably the bushing of hook driving shaft too. But it depends on how the machine is set up. Back in 2015 when set up my long gone 133K I started this topic:

why is the bottom thread under the latch of the bobbin case? Always good to read the manual fully (no offense) - I guess you have a manual, right?

The resistance probably is when the foot lifts up cause it then is working against the foot pressure spring. The higher the foot lift is set and the higher the foot pressure is the more resistance you feel. Just my thoughts. I´m not in front of the machine.

is it just me or does the cam track look concave? It should not look like that, right?