Mr. Jacobs, a onetime amateur boxing champion who had a short professional career, began working with young boxers in 1970 at a recreation center near his home in Palmer Park, Md.
The recreation center could be seen from the front porch of the family home of Cicero and Getha Leonard. A 14-year-old Ray Leonard followed an older brother, Roger, to the gym. He was introduced to boxing by Mr. Jacobs, launching a career that made Sugar Ray Leonard perhaps the country’s most recognizable boxer after Muhammad Ali.
Leonard and the other fighters were up at 5:30 every morning to run, and Mr. Jacobs was sometimes forced to devise makeshift training methods.
“I actually thought he was crazy,” Leonard said Tuesday in an interview. “He had us jab, jab, jab, from one end of the gym to the other. These are memories I will never forget.”
In the early years, the facilities in Palmer Park were so meager that Mr. Jacobs’s didn’t even have a ring for his young boxers: They sparred on a basketball court.
In time, a ring was built, and Mr. Jacobs became a full-time employee at the Palmer Park Recreation Center, leading his young charges through boxing’s time-honored rituals of jumping rope and building coordination by hitting the speed bag.
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Drawing mostly from the surrounding Prince George’s neighborhood, Mr. Jacobs and an assistant trainer, Janks Morton, built an amateur boxing dynasty, producing four straight national Amateur Athletic Union championship teams in the 1970s.
Mr. Jacobs and his wife, Memphis, held bake sales to raise money for the boxers to travel to tournaments throughout the country.
“It was a community thing,” he told The Washington Post in 1977. “We never thought that we’d be going to the Olympics. We never thought how far they’d go in tournaments or even if they’d stay in the gym. But once a young man gets involved in boxing, it’s hard for him to quit.”
Mr. Jacobs trained hundreds of young fighters through the years, but his greatest protege was Leonard, who won 145 of 150 amateur bouts under the guidance of the trainer he called “Jake.” Mr. Jacobs toyed with calling his young fighter “Honey Ray,” but ultimately he was nicknamed in honor of Sugar Ray Robinson, a champion boxer of the 1940s and 1950s.
In 1976, Leonard became an international star when he won the gold medal in the light welterweight division at the Summer Olympics in Montreal.
Mr. Jacobs continued to work with him off and on throughout Leonard’s professional career, which included championships in an unprecedented five weight classes.
“Jake deserves it all,” Leonard told The Post in 1977. “We do the running and take the punches, but he’s all dedication and sacrifice. Putting discipline into kids, taking them off the streets. He’ll show you the basic fundamentals and techniques and then it’s up to you to master them. He’ll do his job, if you do yours.”
David Jacobs Jr. was born May 1, 1933, in Washington. His father was a chef, his mother a housekeeper at congressional office buildings.
Mr. Jacobs won amateur championships as a featherweight boxer and had a brief professional career in the early 1950s.
“I could have gone a long way with a steady coach,” he told The Post in 1980.
He later worked as a delivery driver for a pharmacy in Northern Virginia.
Mr. Jacobs was AAU coach of the year in 1974 and was the head coach of the U.S. boxing team at the 1975 Pan American Games in Mexico City. In 1979, Washingtonian magazine named him a Washingtonian of the year, and his was inducted into the Maryland and D.C. boxing halls of fame.
Survivors include his wife of 66 years, the former Memphis Cobb of Palmer Park; eight children, Gwendolyn Davis of Brandywine, Md., Cynthia Neal of Hyattsville, Md., Diane McGlone, Dorothy Jacobs and Daveida Jacobs, all of Palmer Park, Harold Jacobs of Clinton, Md., David Jacobs III of Washington and Alphonzo Jacobs of Largo, Md.; a sister; two brothers; 20 grandchildren; and 18 great-grandchildren.
Mr. Jacobs continued to train boxers at what is now the Sugar Ray Leonard Boxing Center in Palmer Park until 2013.
Leonard, who now lives in California, had not seen his mentor for several years before making an unexpected visit to Mr. Jacobs’s home in December.
“He opened the door,” Leonard said. “We both hugged each other. He was the man who inspired me.”