The last time the Washington Capitals visited Boston, in April, they rode Bob Kelly's game-winning goal and Mike Palmateer's acrobatic goaltending to a 3-2 victory, their first at Boston Garden in 16 tries.

The Capitals will be back tonight, hoping to repeat that triumph and set a pattern for a successful season. It obviously is a difficult assignment, irrespective of the Bruin's 7-5 loss to Quebec in their opener at Boston Garden on Thursday.

In that one, Boston goalie Rogie Vachon managed to stop only 14 of 21 shots-headed his way. That subpar effort lent support to the contention of many critics that the Bruins are showing signs of age, with a lineup that includes Vachon (36), defensemen Brad Park (33) and Dick Redmond (32) and forwards Wayne Cashman (36) and Don Marcotte (34).

The Bruins were in Hartford last night while the Capitals were entertaining Detroit, so both teams figure to be a bit weary for the 7 p.m. contest. Washington had hoped to increase its opportunity for sleep by chartering to Boston after last night's game, but no flights were available, so instead the Capitals will fly commercial this morning.

This game marks another first of many for Bobby Carpenter, who scored a goal in his NHL debut at Buffalo Wednesday, then gave the home fans a look at his multiple talents last night. Carpenter is from Peabody, just outside Boston, and he was a star for St. John's Prep, the Massachusetts champion in Carpenter's sophomore year.

The Capitals had been concerned, up until the moment they drafted Carpenter with Colorado's help, that Boston would engineer a deal with either Los Angeles or Colorado and gain the rights to the youngsters.

Hartford feared the Bruins' intentions, too, and the night before the draft in June, General Manager Larry Pleau told the Whalers' broadcaster, Chuck Kaiton, that "I just have a funny feeling we're not going to get Carpenter."

Pleau was shocked by the Capitals' move and said recently, "I never dreamed Washington was after him. I just somehow figured somebody else would beat us to him. He's a smart hockey player. He has a good head on his shoulders and he knows the game. He's mature for his age. He'll be a good one."

Just as Hartford figured Carpenter would sell tickets and Washington hopes to capitalize on him at the gate, Boston could have used the youngter's ability to pad its attendance.

During the 1980-81 season, crowds at Boston Garden totaled only 445,598. That was a drop of 49,000 from the year before and was 140,000 fewer than in 1974-75, when Bobby Orr was the principal attraction.

Evidence of the falloff in interest in the Bruins is even greater on the television front.Channel 38 recently reported that 22 percent of the TV audience watched the Bruins from 1970 to 1976. Last season the figure was 7 percent. Carpenter could have helped the situation over the full season. Instead, this is the first of only two appearances for Carpenter on his home turf.