Well, I picked up a 1983 c900 8V non-turbo just yesterday. The car had been running very rough for the previous owner, yet when I went to pick it up it ran great! He stated that prior to parking the car sometime last december it was running as though on 2-3 cylinders and when he would give it a little throttle it would sputter and not build any revs.
Well, I was a little optimistic because the problem seemed to be intermitent. I replaced some vacuum lines and check some things over and it was still running well - until I stated it up to drive it into the garage...
The car starts OK, and it idles OK, but when you press the accelerator it sputters and will NOT build revs! If you give the accelerator a sharp, quick press it will rev a tiny bit until it sputters. The most I could coax out of the car was 2k RPMs.
Ok all of my C900's are 8V's with CIS (and none with APC). First thing is to make sure you've got good fuel in the tank - 8V's should run on 97+ fuel (any of the 98 octane fuels providing they have no significant ethanol content are ok). Run a good injector (or generic fuel system) cleaner through the fuel system next time you fill the tank.
It might seem obvious, but check all of the ignition leads to make sure they join to the plugs and the distributor cap properly, and associated wiring to the rest of the ignition parts. If you have thoroughly checked all the vacuum lines (especially the ports on the intake manifold - the rubber bits shrink and get hard/brittle with age), check all the electrical connections to the warm-up reg, etc. though they'll all be having no effect after the engine is warmed up.
If you think the fuel filter is ok, leave it alone, but see if you can find out from the last owner if/when it was changed. What is the coolant and the engine oil like? Do they look contaminated (which might indicate problems in the cylinder head)?
An easy way to check the basic CIS system function is to remove the fuel-pump relay, and using a fused switch connected between pins 30 and 87 of the fuel pump relay socket, remove each pair of injectors (or both pairs if you have a big enough pan/dish) by undoing the big bolt between each pair, place them carefully in a pan taking care with the plastic fuel lines, and when they're in a pan, remove the air snorkel to get access to the airflow sensor plate and lift it gently with pliers while you have the fuel pump manual switch turned on.
You should get an even spray pattern of fuel from each of the injectors. While they're out, look at the rubber injector seals as they will also dry out, shrink a bit and tend to break down over time.
If you get an uneven spray pattern the injector affected might be blocked, or the fuel distributor might not be working properly (though it's usually going to be a blocked injector if that occurs). That's why you should first of all try a good injector cleaner additive and do more tests later on.
Be really careful when you do the injector spray test - don't do it with the engine warm or hot to reduce the risk of fire, and make sure you do it with the car outdoors or in a very well ventilated building.
You can do pressure tests on CIS (the Bentley book is very good with the descriptions) but you need a special test kit which connects into the line running between the warm-up regulator and the fuel distributor to measure the various pressures.
There's no reason why you can't take the fuel pump out of the tank to check that the rubber part of the fuel pickup hasn't turned to goop, but undoing the clamp holding the pump assembly in the tank needs very careful use of a long straight screwdriver or a special screwdriver tool with a movable head.
Another thing you can do is start the engine and just try fiddling with the throttle valve to see what it does to the engine. That way you can stand next to the engine and check out it's behaviour.
BTW, when you have the snorkel off the air box, have a look inside the throttle valve and make sure it's clean. If you can, get the car to a good mechanic who knows about CIS (preferably a Saab person) if you're not able to figure out what the problem is.
Good luck!
Craig.