What ARE you talking about? And I don't mean this is the nasty tone, kinda way.

Perhaps you need some basic gasoline engine theory reading.

Let's start with this and your other post. (no need to repeat posts, btw)

You took your valve (cam) cover off and you see "carbon"? Like hard, black carbon or goopy gel like brown crud, or what?

You took some compression readings (as the other asked) Did you do this correctly? If you have carbon build up in the combustion chamber and all other things are nominal, you would have HIGH compression readings.

A poorly maintained engine can have sludge in the top and bottom end oil systems.

So let's work with this a bit:

Volvos can suffer from carbon build up in the comb. chamber. There are cleaners and methods out there. BG44K, Fuel Control, Amsoil PI. I can even recommend some cocktail bombs.

If you have sludge in the engine, then AutoRx is the way to go.
--
Paul's Amsoil and other lubricants

Adding to Don Foster's comments... Was your compression test done with engine warm, all spark plugs removed, and throttle blocked wide open?
If not, it's worth doing it again.
--
Bruce Young,
940-NA (current)
'80 GLE V8 (Now gone)
'83 Turbo 245
'73 142 (98K)
'71 144 (track modified--and going to be crushed unless...)
New 144 from '67 to '78
Used '62 122 from '63 to '67

I don't see how carbon on the inside of the valve cover would affect compression.

Do you know what the compression figures for your engine should be? If you have a B21F motor, then it runs lower compresion than what we had here in Australia, so I can't help you there. 105PSI sounds low to me, but I can't really comment.

Carbon increases compression when it builds up in the cylinder head or on the piston crowns, because it reduces the available space
--
Drive it like you hate it

Carbon buildup (that you're thinking has changed the c/r) and varnish buildup (that you're seeing under the valve cover) are different and independent issues.

For carbon buildup in the combustion chambers, you can try an Italian tuneup.

When you're measuring the c/r, be sure to also do it "wet." That is, squirt some oil in each cylinder before you measure. This seals the rings. If the pressure increases significantly in one cylinder (vs. that others), then you have bad rings in that cylinder. If not, then the c/r loss is more likely due to a leaky valve.

For the varnish buildup inside the engine (under valve cover), you can try multiple, repeated oil changes with synthetic. Synthetic oil has much higher cleaning properties than ordinary oil, and it will cut varnish quicker. But keep in mind that it will also loosen sludge and crap that is otherwise solidly deposited in your engine -- and that crap will start to float around. So you should be prepared for dumping the oil often as the sludge and "mud" gets stirred up.

And synthetic can loosen the varnish that has hardened around engine seals, causing them to sometimes weep.

Unfortunately, aside from the Italian tuneup, there are no easy answers.
--
Don Foster (near Cape Cod, MA)