Gasoline prices at the pump have jumped 8.1 cents a gallon since Memorial Day--the fastest upturn in prices since the period immediately following decontrol, according to the American Automobile Association.

Nonetheless, prices are still about 6 cents a gallon lower than they were a year ago and are not likely to rise much higher for the balance of the summer, according to industry observers.

According to the AAA's gasoline-price survey, the average price of gasoline as of June 29 was $1.314 a gallon ($1.339 for full-service regular, $1.391 for full-service unleaded, $1.23 for self-service regular and $1.296 for self-service unleaded).

Washington-area prices were slightly higher, the AAA said. The average price was $1.322, while customers could expect to pay $1.391 for regular gasoline and $1.439 for unleaded at a full-service station, and $1.20 for regular and $1.257 for unleaded at a self-service station.

The increases, which began last April, followed a year of plummeting gasoline prices, with both the oil glut and conservation by motorists pushing prices down. "That which had fallen over many months half-recovered in a couple of months," said Dan Lundberg, publisher of a newsletter that reports on gasoline prices.

"By the beginning of this week, the market seemed to have been cleared," Lundberg said. "There's sort of an equilibrium of supply and demand. The market seems to have hit a plateau at both the wholesale and retail levels.

"Since there are ample supplies for the summer and no reappearance of the glut, the equilibrium is likely to continue through to the end of the summer," Lundberg said.

Despite prices lower than last year's, Lundberg said he anticipates no increase in consumption.

"What we've got is price-stimulated conservation," Lundberg said. The recent increase in prices of approximately 9 1/2 cents since April is reinforcing conservation, and conservation is serving as a damper on further price increases, he said.

"Motorists, in more than one way, are in the driver's seat," he said.

The AAA said that the recent price increase was surpassed only by the 13.8-cent jump that occurred immediately following decontrol of prices in January 1981.

The automobile association said that more than three-fourths of all service stations plan to be open all day and during the evening during the July 4th weekend. Another 16 percent will be open 24 hours, the survey found.

The AAA found the lowest statewide prices in New Jersey, where the average was $1.235, and the highest in Mississippi, where the average was $1.387.