About 17,000 volunteers turned out to help Detroit police and firefighters prevent an outbreak of Devil's Night arson as a dusk-to-dawn curfew for youths under 18 was continued through the Halloween weekend. Police had no reports of injuries after scattered fires Friday night. Setting fires the night before Halloween has been a custom in Detroit. Mayor Coleman A. Young said Friday night the number of fires appeared to be a "great improvement" over the previous year. Fire Department statistics show 386 Devil's Night fires set last year. There were 810 in 1984, the worst year on record.