Thanks for the responses - my bad, it's a 1977 CB550F.
I believe it had pod filters at one point because the choke lever was bent and the jets were different from what came in my rebuild kit. By the time I got it, it was back to mostly stock - stock intake box but missing the top cover piece under the seat, so looking down you see the filter.
My rebuild kit included a #100 main jet and #40 slow jet. I have a 4 into 1 exhaust. Also all of the rubber boots that connect each side of the carburetor are new and the clamps are tight.
The slow jet is brand new but not a genuine Keihin so maybe that's the culprit? I still have everything I removed during the carb rebuild, so I can try putting those back in.
I will go through the ignition system first, thanks Dave. It has a Dyna S ignition kit but stock-looking coils. I don't have a timing light but I'll get one and check that, assumed it was set properly and didn't need adjustment but we all know how assumptions go...
I'm measuring with one of these vacuum gauge kits, which I 'calibrated' (just balanced at a few different points) using a vacuum pump at work. I know it's cheap but it's definitely accurate enough that I know I have a problem.
So to recap, I'll confirm the ignition timing is correct, then do a compression test, then replace the plugs & plug wires, then try swapping the other slow jets back in? Hesitant to fire the parts cannon on plugs and wires given that I'm equally low on all 4, it's likely not an individual component issue...