THE Japanese may have gotten the first jump, but United States automakers insist they're still on the right track when it comes to developing a new generation of fuel-efficient engines.Early this month, Honda and Mitsubishi announced plans to produce so-called "lean-burn" engines which they claim will boost fuel economy by 10 to 20 percent. The engines use sophisticated new computer controls and other hardware systems to maintain power while reducing the amount of fuel metered into the engine. Honda plans to introduce a lean-burn engine into the US market this fall. It will be offered as an option on the newly updated 1992 Civic. For now, Mitsubishi only plans to use the lean-burn engine on vehicles sold in Japan. But lean-burn technology is not without some problems. The lean-burn engines are more complex, more costly, and they have trouble meeting strict new auto emissions standards because they produce high levels of oxides of nitrogen. The only current way to meet US standards is to keep lean-burn engines small enough to reduce their overall volume of exhaust. To Francois Castaing, Chrysler Corporation's director of engineering, that is an unacceptable solution. Chrysler could gain access to lean-burn technology through its affiliation with Mitsubishi, he notes. But he adds Chrysler is not going to do so. "Having a small lean-burn engine too small to move [Chrysler's typical products] is probably not a good idea for our customers," Castaing said at the University of Michigan's annual automotive conference in Traverse City, Mich., last week. Instead, Chrysler - and the other members of the Big Three - will follow their own route to fuel efficiency. The most likely candidate is the 2-stroke engine. It uses about 40 percent fewer parts than a conventional 4-stroke auto engine, is far less complex than a lean-burn engine, and theoretically can yield 30 to 40 percent better fuel economy. Like the lean-burn engine, however, 2-strokes still have trouble meeting emissions standards. Big Three executives also want to make sure the 2-stroke will prove durable enough to satisfy customers before they schedule the engine for use in future products. Castaing insists that Chrysler is making inroads. "We have made steady progress," he said. "We hope we are on our way." Insiders suggest that if that progress continues, the first Chrysler 2-stroke could make its way into the automaker's next-generation small car by the mid to latter part of the decade. Chrysler could be beaten to market, however, by either General Motors Corporation or the Ford Motor Company. Those two carmakers are developing their own 2-stroke designs based on licenses granted by the Australian firm, the Orbital Engine Company. Industry insiders who follow the 2-stroke foot race suggest that Ford may be closest to market, and could put its first 2-stroke into a small car for sale in Europe by mid-decade. Ford chairman Harold Poling refused to confirm that goal, but he said Ford is serious about developing a working 2-stroke as soon as possible. But like Castaing, Mr. Poling said Ford will not go into production until it is satisfied the 2-stroke will meet customer and corporate expectations - as well as government mandates. And for the moment, Ford's prototypes are not doing as well as expected. "It is not on schedule," Poling said. But he added that "I would not say it's significantly deteriorated in timing." Some industry skeptics wonder whether the Big Three want to play down the 2-stroke (or any other fuel-efficient engine design) for the moment. If it appeared ready for production, it could influence the fuel economy debate in Congress. Federal lawmakers would like to boost the Corporate Average Fuel Economy standard from the current 27.5 miles per gallon to 40 m.p.g. by the end of the century. The automakers are opposed to such an increase, and a working 2-stroke would belie their claims that the 40 m.p.g. target is technologically unworkable.