Maybe it's a cassette?
On a 12-speed road bike, it's 95% likely that it's a freewheel.
Dirty sprockets are dirty sprockets
True, but it's the pawl mechanism
inside that needs cleaning, not the sprockets.
Thanks for your replys.I removed the rear wheel,disassenbled the deraillur cogs,cleaned them and the chain with some 409,reassembled and now things seam back to normal.
I was referring to the cogs on the freewheel, not the jockey wheels on the derailleur. 409 is a terrible thing to use on moving parts. I hope you thoroughly rinsed the thing out with hot water and lubricated the jockey wheels with chain lube. If not, you'll ruin them very quickly. Same goes for the chain.
Can the power links be installed without a chain tool?
No.
Will I damage the chain by forcing off a link to install the power link?
Force it off with what? Anything you force can get ruined. If you don't know what you are doing, go buy a book on bike repair and a chain tool. Even a chimpanzee can figure a job like that out.
Chains can be well cleaned without removing
No, they can't. You can buy a chain cleaning gizmo, but all it's going to do is clean up the surface of the chain, and spill greasy solvent all over the place. The only proper way to clean a chain is by removing it, putting it in a bottle with solvent, and agitating. Then rinse 3-4 times, dry, and relube. BTW, use only citrus solvents on chains that will be lubricated by wax lubes.