Carb screws should be set the same on the SJ. No offsetting front and rear. High speed, 1 1/8 - 1 1/2, +/- 1/4, low speed 7/8 +/- 1/4 so sayeth the service manual. Instant shut down on throttle opening means it's lean. It's taking one big gasp of air with not enough fuel to go on. Open the low side up to 1 1/8 to start, always start off rich and lean out from there until your idle is responding. There should be no hesitation or gasp and then rev up, that's lean, but there should be no bog or buhhhh and take awhile to clear out, that's rich. Snap the throttle, it should rev up cleanly and instantly, then return to clean idle near instantly. If it hangs at high idle, it's lean, if it sinks below idle and slowly clears out coming back to proper idle, it's rich. And lastly, your carbs need to synchronized before you can even start with fiddling with the fuel screws, just set them both the same to whichever stock setting you prefer and focus on synchronizing next.
A very, and I do mean very, basic starting point is to use the shank of a decent drill bit...TINY drill bit lol, not the cutting fluted side, the smooth shank. With your idle screw, open the throttle way up so you can put the drill bit shank in the throat of carb 1 (Mag/stator side) without the butterfly touching the drill, back the idle screw out until the butterfly just barely touches the drill shank. Check carb 2 to see if the butterfly aslo touches the drill to the same amount. If not, adjust the coupling link screw between both carbs until carb #2 (PTO side carb) matches the same touching weight as the #1carb did. Once done, back the idle screw way out closing the butterflies, then screw back in just to get a little line of opening at the base of the butterflies. This will get you close to calibrated. The reason for the drill bit is because it is the closest thing you will get to a readily available precision ground dowel pin to do a hardware based sync. To really fine tune the sync, you will need a vacuum gauge. I use this one, it has been bulletproof in performance...
https://vwparts.aircooled.net/Snail-Type-Syncrometer-Carb-Sync-Tool-GERMAN-SK-p/ste-sk.htm ...get you one of these and sync your carbs properly. The above method will only get you close. Hope this helps
Quick add, in regards to the photos...I have seen that schmutz before hanging around the flywheel. In my experience it has turned out to be nothing more than someone used excessive assembly grease in the crank seal and or starter bendix. It most certainly can be a leaking crank seal, the only way to know is to pressure test the crank housing. When you get your carbs setup, if the problem continues then a pressure test of the lower end may be the next order of business.
