Sometime around the model year of your car (a '91), Volvo changed from a system using the distributor to adjust ignition timing to a system in which the ECU makes all timing decisions (advance or retard) -- along with it, it introduced a new sensor (to tell the ECU the crankshaft's angle) using a magnetic pickup on the bell housing above the flywheel called a CPS (Crankshaft Position Sensor). More explanation at the end of this.

Honestly, I'm not sure which model year it changed -- maybe '90, maybe your '91. If you have the "old-fashioned" distributor, yes, you can adjust your timing (but you should adhere to the instructions using a timing light -- don't advance it by trial and error).

If you have the new system, it's impossible to change your ignition timing. As I wrote above, the timing is entirely under the control of the ECU. Even if you manage to somehow circumvent the stopper that ordinarily keeps your distributor from being rotated (this stopper was added along with the new system), that will not change your timing at all. But then again, your timing can't be wrong, so you'll have to look for another cause of your problem.
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FYI, the old-fashioned distributor (sometimes) had an air pressure/vacuum diaphragm to use manifold pressure (for load) and centrifugal weights and springs, all to adjust timing based on the need of the engine, as well as (usually) some "hall pickup" to tell the ECU or ignition computer where the crankshaft is. And you could adjust the "basic" ignition timing by loosening a clamp and rotating the distributor body, which would change the timing of the spark in relation to the crankshaft (actually if you want to be a stickler for details, to the intermediate shaft on red-blocks, which are tied to the crankshaft by the timing belt).

But with the new system, the distributor is mostly empty. No vacuum diaphragm, nor any centrifugal weights and springs! Nor are there any "hall pickups" or other devices to convey crankshaft position. Only the rotor remains inside, because the only job left for the distributor is, literally, to direct the spark energy to the right spark plugs. And it's locked in place, so it cannot be rotated.
All of the distributor's old tasks such as deciding the amount of advance or retardation, measuring engine load (based on manifold pressure), and rpm, and even the crankshaft's position, is measured by other sensors, and all of that information goes to the ECU. It alone then decides when to fire each spark plug, based on algorhythms that take all those factors (not to mentions others such as coolant temperature, etc.) into account.
If you did try to turn the distributor's body, all you would wind up doing is interfere with the overlap of the rotor and the distributor cap's electrical contacts.

Hope this explains things.

Good luck.