Twenty years ago auto engineers talked about the "welded hood" or "sealed engine" car. That was to be an automobile where the engine would need neither maintenance nor things like spark plugs and oil for the life of the vehicle.

Such a vehicle is still in the realm where you find the mythical Unicorn, and the idea is one that may never come to pass. But the movement toward it is positively dramatic.

Quietly, but seriously, a number of solid, sound engineering changes in recent years have hugely boosted the periods between normal "scheduled" maintenance. And in the next two years, striking progress in the direction of the welded hood car is in the works.

The premier example: all of General Motors Corp.'s 1979 model cars - they will go on sale in 13 months - will no longer have carburetors where the air-fuel mixture can be adjusted with a screwdriver.

The Environmental Protection Agency in the meantime has been mulling, and is now ready to promulgate, limits to any possible engine adjustment. GM's '79 carburetor change is the most dramatic change that will result in anticipation of such a move.

The Ford Motor Co. is understood to have what it calls a "tamper proof carburetor" in the works that will probably closely resemble GM's carburetor - but there is no one at Ford ready to talk about it who is in a position to discuss the topic.

Chrysler Corp. and others are also known to be working on the topic - and the EPA is expected to issue its formal rules on the issue within a few weeks.

For the average American mechanic and shade-tree handyman, the 1979 GM carburetor will be one of the biggest changes seen on new cars in many years.

General Motors has told the federal government that the air-fuel mixture adjustment on all '79s will be accessable only by completely removing the carburetor, knocking out two metal pins with a punch and hammer, and removing a case-hardened steel cover.

After all of that, it will still require a wrench and not a screw driver to adjust the air-fuel mixture.

Behind the change is the federal government - with the substantial acquiesence of GM. The emissions from cars two, three and more years old have not been meeting the projections of either the government or the automakers. A prime reason is the fact that they are rarely kept maintained as the makers want.

Earlier steps by the automakers have included narrowing the adjustment ranges by installing, for instance, limiting tabs on things like the air-fuel mixture screw. Such moves and such tabs have been soundly and roundly ignored and violated.

With the steady growth of electronics on the automobile - the ignition without points, the more precise control of fuel and engine behavior - large improvements have been made in drive train life.

Most of those improvements have been lost for a variety of reasons; endless talk about emissions; catalytic convertors that require unleaded gas; the seemingly unending year-by-year rise in new car prices; and buyer expectations that always outrun product improvements.

But the fact is that cars go much longer between oil changes; mileage is rising each year faster than it ever did; and while looks, styling and zero-to-sixty performance is not the game any more, there is a new generation of automobiles enroute to u.S. dealer showrooms.