Austarion

My Jap spec GSRIII seems to be running pretty poorly at low revs. Idle is rough and seems to miss and be down on power up to about 2500rpm. Once it comes on boost it seems to run a lot better. I've checked timing, coil, plugs, leads, replaced fuel filter etc. AFM seems to be working? Its a standard motor on the factory ecu with FMIC, TD05 and Cam.
Any ideas?

type A or B injection?

Not sure? It's an '83 Jap spec, factory intercooled and has the oval shaped inlet on the intake manifold.

A.

Look for vacuum leaks. Check pipe that goes to PCV valve.

Had a good look for leaks but no luck. The idle is steady (ie not surging), it just misses at idle and idles quite low, then feels really sluggish until it comes on boost. Almost feels like it's running on 3 cylinders at low rpm?

I have similar probs with my standard JB (type 2 injection), have replaced fuel filter and injectors, replaced most of the ignition system and these things have all helped but I now suspect the fuel pump.
Have you checked or replaced your pump?
I suspect a tired fuel pump and alternator combine for poor running at and around idle (especially in winter), then the pump recovers once the alternator is spun up and voltage is restored, put a charger onto you battery and see if it needs a top up?

These cars are getting old, just about all original perishable parts are due (or past due as you have found out) for replacement.

burnt out exhaust valve? do a comp test..

Did a compression test and found 2 and 3 were down. Turns out the head gasket had gone between cylinders. All sussed now =)