Bartel E. Davis, an 18-year-old accomplice in the murder/robbery of Federal Trade Commission economist Stephen Marston and the serious wounding of his wife last Jan. 2 in their Parkfairfax home, was sentenced yesterday to life plus 70 years in prison after a dramatic hour of courtroom testimony and maneuvers.

To the surprise of Alexandria Circuit Court Judge Albert H. Grenadier, Davis interrupted the sentencing hearing by standing to inform the court that he wished to change his guilty plea.

"Your honor, I want to change my plea because I feel I knew nothing about the shooting," he said in a strong and unwavering voice. "I knew nothing about the gun."

Last June, Davis and his 20-year-old brother, Boggess Davis, and codefendant Jeffrey Harris, 17, pleaded guilty to killing Marston, 36, shooting Martha Riche, 43, and to an assortment of robbery, burglary, and weapon charges. Boggess, who actually shot the couple, was sentenced to two life sentences and 66 years. Harris was sentenced to life plus 80 years.

Earlier courtroom testimony by witnesses had deeply implicated Bartel Davis in the shootings that began as a robbery to get enough money for Boggess Davis to take his girlfriend to the movies. Yesterday, he said he never entered the Marston condominium at 3603 Greenway Pl. nor received any of the money and items taken from the couple.

Grenadier, noting Commonwealth's Attorney John E. Kloch's objection to the plea change, denied Bartel Davis' request, explaining that he had been satisfied when Davis first entered his plea that he "knew exactly what he was agreeing to."

Throughout the sentencing hearing, Davis, dressed in the Alexandria Jail uniform of blue shirt and slacks, persistently challenged the prosecution's versions of what part he played in the crimes.

Davis disagreed with much of a presentence report, a document detailing a defendant's background that is routinely used to assist judges in deciding sentences.

Stanley Davis Jr., Bartel's oldest brother, who testified on his brother's behalf, was removed from the court after shouting from his seat that parts of the report were a "damn lie."

The report described Bartel Davis as a heavy drug and alcohol user who was easily influenced by others. The report said that Davis received little supervision from his parents after they separated when he was 5 years old, leaving much of the child rearing to Boggess Davis and a sister.

"I loved my brother," Bartel Davis told the courtroom.

He said when Boggess and his friend Harris asked him to come with them to a party the evening of Jan. 2, he did not know they actually had planned a robbery instead. Contrary to earlier accounts, Bartel Davis said he did not see a gun until after he heard shots fired in the Marston home.

But Kloch maintained that Davis shared equal responsibility in the crimes and that the only "mitigating factor is that he did not pull the trigger."

He said Davis, as demonstrated in two juvenile convictions for robberies in 1981 and another about a month before the Marston murder, was inclined to violence and was probably beyond rehabilitation.

"I think simply for the protection of the society, this defendant needs to be incarcerated," Kloch said.

Judge Grenadier agreed. He gave Davis 60 years for malicious wounding and burglary and two robbery counts to run concurrently with a life sentence for the Marston murder. He also gave Davis 10 years for three firearm violations to run consecutively with the other sentences.

Davis also received 16 years for two earlier robberies he was convicted of as a juvenile to run concurrently with the earlier sentences.

He will be eligible for parole in 1998.