I think i have decay. inside the capped one is infected can i use amox-clav to help it?

Best to see Dentist: A dentist may put you on antibiotics, but this is not the treatment which is needed to resolve your situation if i understand your issue correctly. Please see a dentist as soon as possible to avoid further pain, tooth destruction and or a possible root canal or loss of your tooth. Most likely an x-ray and dental exam will provide information to resolve your situation.

Shouldnt : You should see a dentist and not self medicate with antibiotics, because if the cause of the infection is not resolved it will come back worse next time.

Temporarily : If there is infection inside the tooth, then an antibiotic can be used to help on a short term basis. This will not be a cure, just a way to help with pain and swelling. The penicillins and similar antibiotcs are the best choice for oral infection. See your dentist as soon as possible.