Constabulary

should be mentioned in the manual - (see picture) loosen the thumb screw and repositioning. This will alter the foot lift - highest foot lift in upper position lowest foot lift in lower position. I don´t know how significant the difference in the stitch length is but to be honest I don´t think this is practical.

Don´t know but maybe your feet are not lifting high enough but that can be adjusted on the rear side on the curve / cam. If the major problem is the shorter stitch length when sewing over thicknesses (don´t know what sicknesses you sew over and how significant this is on your projects) I would say you will have that with any other triple feed / needle feed machine as well cause the needle bar rock frame has a high pivoting point and therefore you have a pendulum effect and the needle is entering an leaving the material always in a certain angle but never in 90°. The thicker the material the smaller the stitch length. The only needle feed machine with a constant stitch length in all material thicknesses that I know of is the BUSMC #6HM (and similar machines) cause the needle enters the material always in a 90° angle. But that machine is probably an overkill for you - for me it was so I (sadly) sold it. Another option may be older drop feed + jump foot machines like Adler 5 / Singer 45K / 132K / 133K. But they have aggressive feed dogs and may mark the leather and many do not have reverse. EDIT: BTW - the Singer 108w is a triple feed too but with a different rock frame (little bit like the BUSMC #6) and the needle is entering and leaving the material in a 90° angle but it cannot sew 1/2" and has no reverse. If I was a sewing machine engineer I would scale up this thingy and add reverse EDIT II: if you want to upgrade from a class 69 I´d say an "affordable" machine could be a 341 Juki clone. You may look for "341 cylinder arm" machines in the UK. But I´m not sure if it would solve the "sewing over thicknesses / stitch length issue" - most likely not. But they use the standard Singer 111 type feet. Interestingly I just figured they seem to sell (sold atm) for approx 1/2 price of the "known" clones on my side of the pond. https://www.sewtex.de/SIRUBA-Naehtechnik-243/FREIARM-STEPPSTICHMASCHINEN--CYLINDER-BED-LOCKSTITCH-MACHINES-246/Cowboy-CB341--Cylinder-Bed-341-Sewing-Machine-1853.html

good lord - to be honest - I´m not sure if it really is worth the time and money (in parts). Maybe leaving it as is is the better option for "just" an exhibition item.

Yes holes are too wide for the chosen thread, I would go at least 1 needle size down - maybe even 2 but depends on the thread and if you use bottom thread 1 size smaller or not. The thread charts you find are rather a rule of thumb, they work under certain conditions but since mentioned thread size is calculated by weight per 1000 meters and not by its actual thickness so you have some variables. So use these charts as hint but not as a fix rule. Cotton thread, Polyester thread and Nylon thread may have a different actual thickness at the same thread size. Regarding needle - believe it or not some years ago I ordered 3 or4 different size of System 328 needle from a brand I do not remember but it was not Schmetz or Groz Beckert. Two where the exact same size but with different needle size stamped on the needle. Not kidding! Maybe it was a production error - maybe not. You know what I mean? That said - you always have certain variables when you choose the right thread and the right needle for a certain project. And of course the type of leather you want to sew matters too. This may be useful for choosing the right needle tip. https://www.schmetz.com/en/industrial-needles/needle-compass/cutting-points/ My favorite leather tip is S-point cause I like straight seem stitches better than slanted. My 2 cents

It´s mentioned in the 153 manual. Not super clear pics but maybe it helps.

when you are bitten by the bug there is no cure - cast iron sewing thingies can become addictive. Guess how I know. But meanwhile I found a way to let some of them go again (but only when other cast iron is knocking on the door) The color reminds me of some "Pink Panther" British SAS desert Landrovers. https://www.pinterest.de/pin/759419555889957308/

Ouuuh - thats an ambitioned project ! Keysew in PA it out of business as I have learned recently. Maybe worth checking with Toledo Sewing Machines in Ohio.

FWIW - when you have skipped stitches it most of the time is the needle bar set too high / needle bar has risen by the time.

He wouldn´t cause he only has one of them ====== Have you checked the foot lift adjuster on your machine and have set the stitch length adjuster to a longer stitch length? Have you read the manual? To start with something - please post some good clear pictures of your machine from front and back.

For what ever reason you may have a top tension release. Do you have a knee lifter you ever so slightly push the knee against when sewing? Or pedal for lifting the foot you rest your foot on when sewing? That MAY cause a tension release. Just a guess... Just to try something... You see the black disc right behind the thumb nut - remove it and put back on the beehive spring and thumb nut and try again. This way you disable the tension release entirely. There is a tiny pin behind the black disc - don´t loose it. Is just to see if there is a change when sewing, it reversible in a few seconds. Have you checked the needle hook timing? Just to make sure the machine is set up for 134-35 needles. Needle scarf is facing toward the hook right?

well, the tension parts on sewing machines have not really changed significantly so I´m sure it is the balance between top and bottom tension you have to figure other than that make sure the top thread comes off the spool w/o hanging and nothing in the top thread path is causing drag or additional tension.

if you cannot find a manual look for Singer 7 manual (flat bed machine instead of cylinder arm). There were some different models but I´m sure one will be close to your Singer 11 in terms of the operational / control parts, threading and so forth.

In 7 years it is well possible that the needle bar has risen a little bit. I´d check the needle hook timing and probably lower the needle bar a tiny bit.

I would check the hook area for thread bird nests so similar. Service manual is available online - maybe it helps: https://www.duerkopp-adler.com/fileadmin/dag/Media/Downloads/176/S_179_EN.pdf

Threading the bobbin case is mentioned in the manual (not super clear pics though) https://www.manualsbase.com/de/manual/578012/sewing_machine/singer/307_g/ Other than that the hook is the same as on 107w and 143w so their manuals work in that regard as well.

I´m not sure if sandpaper is the best solution. For obvious reasons the last thing you want in your machine is sand. Better use thin felt discs or bobbin case stars https://www.college-sewing.co.uk/catalogsearch/result/?q=490468+ I´m also using stars and older type Singer springs - not sure if they are still available: https://leatherworker.net/forum/topic/88778-singer-153w101-bobbin-case-is-stuck/?do=findComment&comment=607136

Guess why I love Singer machines and Singer based machines so much However the linked Adler feet are made by Kwokhing which is a quality manufacturer and the linked Singer feed are of unknown make as it seems.

Dürkopp-Adler has reworked their main website and a lot of older content is no longer available. Figured this with their formerly available "Sewing Lexicon". Many manuals are available here (but not all): https://www.duerkopp-adler.com/downloads

most likely for luggage making. maybe worth checking the Ohio Travel Bag website maybe you find something similar https://ohiotravelbag.com/

Well the ancestors of this machine are the Singer 153w for the bottom part and the Singer 211 for the top part. The stitch length lever is of SEIKO origin I guess. Nowadays Seiko is still making machine that look similar the bottom part I´d say is Seiko CW Series and the top part Seiko LSC series to stay in the cylinder arm section (flat bed would be LSW Series): https://www.seiko-sewing.co.jp/en/model/cylinderbed/ A nowadays clone is the Consew 287RB - you can be sure it is almost the same machine (if not entirely) as yours - maybe with minor differences - maybe. You´ll find an instruction manual and parts list in the below link: https://www.consew.com/View/Consew-Model-287RB-2 You can be sure most of the parts for your machine are still available cause a lot of the clones are based on older Singer machines. Same thing with the feet. Look for Singer 111, 211, 153, Consew 227, Seiko CW, LSC, Juki 562, 563 and many many others are still using the same Singer feet that your machine is using as well. Hope this helps

slightly larger I wanted to write

looks like your needle bar is missing the tread guide. I would also try lowering the needle bar a tiny bit even when you think the timing is right. That makes the thread loop a little bit wider when the needle rises - worth trying I´d say. Furthermore - you have installed a hook for heavier thread. There is a variant of this hook for lighter thread as well. Not sure at what thread size it is recommended but it is an option. The difference is a small raised tip on the hook gib. But different hook would be the last thing I would try. See pictures Have you tried heavier thread in the bobbin case - meaning #92 top and bottom? Maybe try a slightly needle like 130 metric.

well, depends on if you need the comfort of this knob and how often you really adjust the foot height. On my machine I have set it to the highest lift and never went back. Usually all triple feed machines have some sort of foot adjustment. Most have some sort of cam on the rear some have a push knob on top of the machine... Just different systems.

You cannot adjust the needle bar height on Dürkopp patchers cause it has no "go through needle bar clamp" Even if it had a "go through clamp" (some CLAES patchers had this) going from shorter to longer needle is easier but from longer to shorter is not always possible due to the given length of the needle bar - you know what I mean? 135x16 needles are shorter than System 88 needles.

You can DIY speed reducers 45mm Pulley 130mm Pulley piece of 15mm shaft (15-20cm long or so) 2 pillow block bearings for 15mm shaft some wood from crap box some screws, nuts & bolts some DIY skills Shaft, pulleys and pillow block bearings are from Ebay. I made one for my Singer 51w setup. But there are many other samples for DIY speed reducers in the forum.