Launching passes that floated like bloated butterballs but stung like killer bees, Billy Kilmer, that old furnace-faced fighter, began still another personal comeback yesterday that ended in a 38-20 Resdkin victory over the San Francisco 49ers.

In his first start at quarterback this season, the 39-year-old Kilmer recovered from a slow start and some heavy RFK Stadium booing to rally his team from a 10-0 deficit to a victory that was assured with two knockout third-period touchdowns.

Kilmer completed 12 or 23 passes for 185 yards and two touchdowns as the Redskins posted the most points since the 1975 season.

But even more important, the Redskins, now 7-2, put a halt to a two-game losing streak and took a one-game lead over the Dallas Cowboys, 6-3, in the NFC East race. They also put more distance between themselves and the third-place New York Giants, now 5-4.

"Sure I was nervous, it was like my first rookie game," Kilmer said. "I was apprehensive . . . I was just trying to keep everybody calm, mostly myself. I just tried to play my game, with no mistakes, and let my team work for me."

It did, splendidly. John Riggins scored two touchdowns on one-yard runs despite sloppy handoffs from quarterback to fullback that hardly seemed to matter.

Kilmer, who had no interceptions, fired touchdown tosses of 50 yards to Danny Buggs and 17 yards to John McDaniel. Tony Green scored on a five-yard, quick-hitting touchdown run and Mark Moseley kicked a 26-yard field goal, for the final points of the day.

Redskin Coach Jack Pardee had informer Kilmer Tuesday he would be starting and yesterday the coach said he made the decision to switch from Joe Theismann "to try and get things turned around - get a spark going in our offense, to get a little better running game going, block a little harder, runners running a little harder."

And the mostly inept 49ers also cooperated.

Quarterback Steve DeBerg tossed up three interceptions. One pass was swiped by Ken Houston in the second quarter when the 49ers were driving and leading by 10, and two picked off by Jake Scott.

San Francisco also fumbled six times, losing two, handed the Redskins and easy touchdown drive with a three-yard punt and ran an aborted fake punt that resulted in a 15-yard fumble that produced still another piece-of-cake Redskin touchdown.

The 49ers did not start out that way. They drove 78 yards the first time they had the ball and took a 7-0 lead on Paul Hofer's two-yard touchdown run and the extra point, then stretched that advantage to 10-0 with Ray Wersching's 29-yard field goal with less than two minites elapsed in the second period.

And now, here they were again, with a first down at the Redskin 19 on their next possession when the Redskin defense finally decided to get serious.

Mike Curtis, making a comeback himself yesterday after four weeks on the bench, blitzed out of the Redskin born-again 3-4 defense and sacked DeBerg for a six-yard loss.

Again the young 49er quarterback dropped back to pass, and he aimed downfield at rookie tight end Ken McAfee. Houston read the play from the start, cut in front of McAfee for the catch and was off on a 29-yard return to the Redskin 44.

"I should have had a touchdown," said Houston, who was tackled by fullback Bobby Ferrell. "The guy who got me was lying on the ground. I guess the old gray mare ain't what he used to be.

"Yeah, it was a big play for us, a turning point, sure. It just seemed like we were sitting there waiting for something to happen. We've been slumping and we needed just a little base hit to get us going. It felt nice to help out again."

It felt even nicer when Kilmer fired back-to-back sideline passes to wide receiver Ricky Thompson for 21 yards to the 49er 22. Six plays later, Riggins just did manage to hangs onto a handoff from Kilmer, bounced off one tackler and eased into the end zone.

The Redskin defense held again and forced a San Francisco punt at the two-minute mark. The Redskins originally had planned to return the kick, with three men deep, but decided during the time out to put on a rush.

"They had a bad snap earlier in the game, and they had two blocked last week, so we decided to go for it," said John Hilton, the special team coach. The 49ers looked confused trying to figure out their blocking assignments, and punter Mike Connell, a rookie, had to field the center snap on one bounce. The result: no block but a three-yard punt out of bouns at the 49ers' 42.

The Redskins took over there, then were faced with a third and 18 at the 50 wih 1:38 left in the half.

From there, Buggs went over the middle on a crossing pattern. The speedy wide receiver, who earlier had a 33-yard touchdown pass nullified by a motion call on Benny Malone, caught Kilmer's throw between four defenders at the 30, cut to his left and cruised into the end zone.

That 50-yard touchdown and Moseley's extra point gave the Redskins a 14-10 lead, and they never trailed again, although the 49ers drove to a 35-yard field goal as the hald ended.

The field goal drive was aided by a rare unsportsmanlike conduct penalty against the Redskins. It was assessed because defensive tackle Perry Brooks was detected calling defensive signals that were similar to offensive signals used by the 49ers.

The Redskins atoned for talking out of turn on their second series of the third quarter - a 60-yard touchdown march that began witha 21-yard pass to tight end Reggie Haynes and ended with Kilmer's 17-yard pass to McDaniel.

McDaniel caught the ball between defenders Vern Roberson and Eddie Lewis, who barely missed making sandwich meat of the Redskin receiver. He made the catch with his back to the endzone and back-peddaled in for a 21-13 lead.

Now it was the 49ers' turn to confuse themselves with some sloppy signal-calling.

Connell came in again to punt, and again the Redskins showed him a 10-man front. The 49ers had called a punt fake in the huddel, but 49er Coach Pete McCulley said the play was supposed to have been canceled when the Redskins went into that defense.

Someone forgot to tell center Fred Quillan, who snapped to ball to his slightly startled upback, Bruce Elia.

Elia was surrounded by swarming Redskins and hit by Gerard Williams as he pitched the abll back to Connell. The lateral sailed over his head, and Connell recovered at eh 49er 19, where the Redskins took over.

A screen pass from Kilmer to Malone carried to one and Riggins lugged the ballin from there for a 28-13 lead with 6:31 left in the third period.

Scott's first interception set up the Redskins' breathe-easy points - Green's five-yard run up the middle on fourth-and-one for a 35-13 lead with 14:14 left to play.