1993 Mazda B-2200 engine cranks but wont start

1993 Mazda B-2200 mileage: 80,000. My car was tuned up in October and the muffler is basically new all around. I started my car the other day and it was very loud and black soot came out of tailpipe. I did not drive it for a week because I was out of town. When I got back the truck would not start, it was not the battery, it turns over but does not start. My air filter gets dirty quickly. I am not losing oil. My mechanic towed it and put new plugs in it and said the motor was going and it was probably the rings. Does not sound like rings because I thought that if there was a ring problem I would be burning or losing oil? What do you think? Thank you.