The Philadelphia 76ers extended themselves in the first and fourth quarters enough to thump the beleaguered Washington Bullets, 120-102, tonight at the Spectrum.

Led by reserves Jim Spanarkel and Steve Mix, the Sixers blew the Bullets away in the fourth quarter, sending them to their fourth defeat in the last five games.

The Bullets were in the game in the middle quarters only because the Sixers stopped running and their shots stopped falling, and because Roger Phegley, Larry Wright, Greg Ballard and Mitch Kupchak were playing as well as they have all season. When it was time to go ahead and win the game, the Sixers had the firepower and the Bullets didn't.

Julius Erving and Maurice Cheeks led Philadelphia with 21 and 20 points, respectively, but neither played in the fourth period as Spanarkel and Mix stole the limelight.

The 76ers went into the final quarter with an 82-75 lead, but outscored the Bullets, 22-9, in the first seven minutes of the quarter and coasted in from there.

Spanarkel had eight points and seven assists in the quarter and Mix had 11 of his 15 points in the period.

Bullet Coach Dick Motta made two changes in his starting lineup. Elvin Hayes sprained his ankle in Tuesday's 96-91 loss to Denver and didn't dress for the game. Ballard started for him and scored 20 points and had 10 rebounds.

The other change Motta put off as long as possible, but Phil Chenier's continuted slump forced the coach to start Phegley at the shooting guard spot.

The second-year man responded with an outstanding performance. He led the Bullets with 22 points and four assists and he provided most of the offense in the third quarter with 14 of his points.

Kupchak, splitting his time at forward and center, played 21 minutes and scored 20 points. He also had three steals.

Wright, who had been used sparingly the past few games, ran the offense most of the game and scored 11 points and had 10 assists and only one turnover in 33 minutes.

Struggling Kevin Porter played only 15 minutes, didn't score a point and had only two assists.

So impressed was Motta with the performances of Phegley and Wright that he said he is leaning toward starting that duo in Friday's game against the Indiana Pacers at Capital Centre.

"I thought Larry ran offense and Roger responded to the pressure well," Motta said. "They could be my starting guards Friday. I've got to do something."

The Bullets tried to run with the 76ers and it just didn't work. Philadelphia controlled the boards and seemed to come up with every loose ball in sight.

"They're so big you have to run them," said Wright. "You can't set up and beat them. I think they just outhustled us tonight, though."

The Sixers came out and blew by the Bullets as if they were statues in blue uniforms.Erving was driving, dunking and passing to open men with ease and the Sixers took a 19-4 lead and barely worked up a sweat doing it.

Every Bullet on the floor missed at least one shot in the first two minutes of the game and the Bullets went scoreless for 3 1/2 minutes.

Ballard scored eight points in the final 3:40 of the period to get the Bullets to within seven points at 25:18 by the end of the period.

With Hayes out of the lineup, Kupchak played forward instead of center and made his first five shots of the second quarter to keep the Bullets within striking distance.

A henry Bibby 20-footer with 6:30 left in the second quarter increased Philadelphia's lead to 43-31.