84 GSL 12a
Stock Nikki and intake
Stock dizzy w/recently replaced NGK spark plugs and wires
Stock exhaust manifold then straight pipe to straight-through muffler

All during the winter while it warmed up, my 12a would stumble all over the place but keep running. Once it warmed up though it kept a good smooth idle.
In the last month even the idle has become very lumpy.
Recently I've tried:
-replaced fuel filter
-adjusted idle speed and mixture screws (followed Sterling's instructions!) and got the idle down to a kinda-solid 600 BUT still the stumble!
-adjusted timing, getting slightly better response now but STILL the stumble.

It keeps the stumble when I throttle up slowly from idle.
It loses the stumble once I engage the clutch - ie. when it's actually doing something.

What do I look for now?
-Accelerator pump?
-Fuel pump?

 

With a 600 rpm idle, I doubt you have any kind of a vaacuum leak, but it is worth checking into as I have found even the slightest leak will affect the idle greatly. It also just seems to be in the rotary's character, as it builds carbon in different parts of the rotor and knocks it loose and burns it off, to idle like **** in the mean time. It can't hurt to look into the items you mentioned above, but I bet a good seafoam treatment is in order. People rave about how well the car runs after doing this. It might be worth a shot.

 

Check this hose.
https://www.rx7club.com/1st-generation-specific-1979-1985-18/hose-question-750125/

 

Wow cheers 74RX4 that's exactly it, I can feel the hole in that hose. Time to get some radiator repair tape.