When the Italians scooted out of the US, the Japanese scooted in

July 24, 1984

Put an otherworldly Grace Jones in the same commercial with fellow rocker Adam Ant and what have you got? For starters, one of the most successful television ads in history.

Throw in another 30-second spot pairing your product with the new-wave Devo group's robotoid performance, and you may have a genuine fad on your hands.

It's too early to tell whether the latest vehicle venture by the American Honda Motor Company - motor scooters - is more than a fad. This year the Japanese manufacturer is spending an estimated $6 million to promote its rakish, wedge-nosed two-wheelers. Honda's target: the youth market, 14- to 24-year-olds.

''The young people are in front of all the trends, like blue jeans,'' explains Michael Howard at Honda. ''The kids wore them, then all of a sudden they became fashionable. The same thing happened with stereo components. In the same way, the scooter trend will spread and grow.''

Maybe so. The ads, which placed in the top 5 percent of all television spots in viewer recognition, according to a recent Gallup-Robinson study, are apparently changing the image of scooters. And cash-carrying customers are showing up on dealership floors.

''It's amazing. We can't get enough of them,'' says an incredulous Alex Sullivan, standing among a dozen scooters decorated with red ''sold'' tags at Honda of Boston.

''Scooters used to be uncool,'' notes Mr. Sullivan, a salesman here. ''I guess it goes to show what advertising can do.''

Honda, which quietly introduced the new product in the United States last year, has sold about 35,000 scooters - a doubling of sales - this year. Another Japanese motorcycle-maker, the Yamaha Motor Corporation, also brought out a line of scooters last year.

Sales of both lines ''have really caught fire this year,'' says Alan Cutter, manager of Moped City, Cambridge, Mass. ''It's the best the scooter business has been since the '60s. We've already sold many more scooters than we sold all of last year.''

In addition to advertising, Mr. Cutter says, both manufacturers have stopped selling mopeds and cut low-power motorcycles from their lines.

For 40 years Italy's Piaggio sold the Vespa and had the largest share of the tiny US scooter market. But rather than meet California emission-control standards, Piaggio pulled the Vespa out of the US in 1981. That left the door open for the Japanese.

As of May, Honda had captured 80 percent of the market, and Yamaha had 19 percent, according to a sampling of scooter registrations compiled by the R. L. Polk statistical firm in Detroit.

Motorcycle dealers are high on the little machines, and with good reason. Sales of motorcycles have been slumping for the last five years. To stay afloat, many dealers have diversified into selling three-wheeled all-terrain recreation vehicles and such ''power products'' as lawn mowers. The scooter is seen as another leg to stand on.

Are scooters cutting into motorcycle sales? Not at all, reports John Brumm, editor of Motorcycle Dealer News: ''The dealers are consistently saying two things: Scooter customers are not motorcycle customers, and you can't put an age on them.''

Despite Honda's youth-oriented marketing strategy, the broad appeal lends credence to the argument that this is more than a fad, Mr. Brumm says. ''I talked to a Riva (Yamaha) dealer in San Francisco the other day. He's attracting young metropolitan professionals and discovered these are wonderful rental vehicles,'' says Brumm.

Yamaha's strategy is to draw the over-30 crowd. The company's $1 million ad campaign revolves around the slogan ''Transportation for People Who Are Already There.''

The scooter has come a long way from its post-World War II origin from leftover aircraft parts. Yamaha's top machine is the $1,700 Riva 180 Deluxe. Honda's flagship model, the Elite, which sells for $1,400, has a pop-up headlight, an LED digital instrument display (as does the Riva), and a foot heater. And it looks like something out of ''Return of the Jedi.'' But the more conventional-looking Riva 180 Deluxe has more power - 80 miles an hour vs. the Elite's 60 to 65 m.p.h., says Mr. Cutter of Moped City.

At the other end of the spectrum, the Honda Spree is priced at $480, less than many mopeds. Finally, unlike their Vespa kick-start, manual-transmission predecessor, all Japanese models have electric starters and automatic transmissions.