I picked up a 1980 hardtail project bike (my first XS650) earlier this summer that was mostly complete but ran rough. The previous owner really didn’t know anything about the bike or its history but It was a good deal so I rolled the dice. The Bike (initial configuration as purchased): 1980 BS34 jetting: 45 pilot, stock/fixed needle,145 main, mix screws 3 out, floats L/R @ 23/24.5mm respectively Stock TCI w/ battery Barebones wiring & kickstart New plugs gapped @ .028” Pod air filters 2-into-1 open exhaust. Problem: The bike starts and idles great (I'm in California, ~400' USL) but severely stumbles 0-1/4 throttle, then above about 2k rpm bike’s midrange hits good and pulls hard through top end. My first thought was a lean idle circuit and possibly needle circuit so I focused my energy there first. Things I’ve done to initially correct/inspect: Fully cleaned & serviced carbs and ensured all passage ways (especially pilot circuits) are clean/clear, diaphragms are intact, pods aren’t covering intake port jets, replaced float valve and seat oring, inspected mixture screw oring, and reset floats to 27mm (I’ve got the brass floats). After re-installing carbs, syncing carbs, test riding, and not getting noticeable improvements (even tried dialing the mixture screw in ¼ turns from 2-3.5 out with no avail) I decided to go back and establish a baseline on everything since this bike’s history is an entire unknown. Good compression in both cylinders, check Good fuel flow, check (gas cap venting properly & inline filter working) Cam chain adjustment, check Valve adjustment done (intake .003”, exhaust .006”) Good battery voltage, check (static voltage 12.9V) Good spark, check (kicked over with spark plug grounded to head) Coil voltage good (0.15V drop from battery to coil) Coil continuity/resistance good (primary= 2.5 Ω, secondary= 12.9k Ω, plug wire to coil core=∞ ) Alternator pick up resistance good (blk-gry=719 Ω, blk-org=764 Ω, gry-gry=1.48k Ω ) Active/live pick-up signal test, good (used LED lights plugged into blk&org pick-up leads to confirm) Regulator/rectifier tested good (A/C measurements wht-wht = 10.6-10.9VAC for all 3 combos @ idle) Charging voltage is good (measured 14.72V @ ~3k rpm) All connections checked & cleaned for corrosion After confirming I’ve got both good fuel, spark, and charging system I tested for air leaks around the known culprits (boots, throttle shaft, etc.) since I still felt a lean condition was a strong possibility. Also double checked the pods aren’t blocking jets (good there too). I tested and re-tested about a half dozen times, each time pulling, cleaning and rechecking my work. Yes lots of beer and cursing! I then decided to try the Canadian needles and emulsion tubes (as a set) on various clip positions (from 2-4 from top, 3 is middle position) with no real improvement. Also tried going up on the pilot jet from 45 to 47.5, then back down to 42.5 all without any real improvement from 0-1/4 throttle (taking off from a dead stop). At this point I have done everything I can think of with carb tuning within ranges of settings that are known to work good and feel like I’ve narrowed this problem down to a single ‘untestable’ component, the Ignition Unit. Or at least that’s my uneducated hunch unless I’ve overlooked some things. When I used my advance timing light (set to 0) to check the timing of my TCI ignition, I get a steady strobe with idle timing retarded by ~1-2° (just to the right of the F mark). But when I try to slowly rev engine it begins to stumble as usual AND my timing light begins to misfire in conjunction with the stumble. Once I get the rpm’s above about 2,000 rpm the engine smooths out (as well as the timing light) and I can accurately make out the advance timing mark as it approaches the fully advanced position. This further leads me to believe the stock Ignition Unit has something to do with the low rpm stumble, not a carb issue. If you’ve read this far then you are a true gear head and for that I thank you. If you see anything that I’ve missed and/or have some input to help get my tuning woes off high-center, I would very much appreciate the input. I’ve ‘read til I bled’ in hopes to solve by search but I am left with the reality that I need some real-time help on this one. Thanks in advance!