Lean surge under light acceleration and cruising conditions?

So, for starters, my car isn't a Justy but a 1991 Subaru Domingo KJ8 with 63,000ish miles (102k kilometers), an imported Japanese van that uses an engine derived from the 1.2L EF12 Subaru Justy engine, using most of the same componentry, just arranged in such a way to fit into the back of the van.

Recently, I've been experiencing a surge under light acceleration and cruising conditions. My carb adjustment screw, idle adjustment screw, and choke all seem to be adjusted properly. I've also installed a new OEM air filter and new fuel filter, which have helped overall performance but have not resolved what seems to be a lean surge. The car idles fine around 1000rpm with no jumps or variances, and under hard acceleration (foot on the floor) it pulls well and has no issues. Further, when out of gear, under slow, even acceleration there does seem to be a little bit of a pause between 2,700 RPMs and 3,100 RPMs, but only sometimes, it isn't a consistent flat spot. But under light throttle the engine jumps and sputters, a sort of "on/off/on/off" feeling. My question is, does this sound like a vacuum leak, and if so what's the best way that I can trace said leak? Or could my accelerator pump possibly be misadjusted (it does have an adjustment screw)? My vacuum advance seems to be clean and healthy, but could that be an issue?

Quick things to note: Air filter is new as of a month ago, OEM spec and adjusted carburetor for airflow difference (old one was really dirty). Fuel filter is new as of 2 months ago. Time belt new 30,000 kilometers ago. Fuel pump is original, doesn't seem to have issues related to my problem but I'm not ruling it out.

This has been plaguing me for 2 months now and it's been incredibly annoying. I appreciate any assistance.