Despite some misgivings that public service jobs may be an expensive - and not very durable - way of cutting unemployment. Congress now seems on the way to approving President Carter's request for $6.1 billion to create at least 750,000 such jobs in the next two years.

The legislators just as clearly intended to raise the ante on Carter's request for $2.2 billion in more public works expenditures in the same period which might create 200,000 more jobs. Already, the actions of the budget committees in both houses point to a doubling of Carter's first-year request.

By and large, labor unions and liberal economists ahev fovored the use of public service jobs, at a time of recession, as a way of providing useful work for young adults, teenagers, and minority-group members who could not find regular jobs.

Businessmen and conservatives, on the other hand, warn that such jobs are "make work" - a distant cousin of leaf-raking in the New Deal days. Moreover, many disinterested academic studies seem to agree that without legislative teeth to prevent it, 40 to 50 per cent of public service jobs money after one year, gets spent for activities that state and local governments would have financed themselves anyway.

And after two years, the "substitution rate" is close to 90 per cent , manpower experts say.

In a report published yesterday, the Congressional Budget Office estimates that the substitution rates may be even higher, adding that "this phenomenon may significantly reduce the effect if public-service employment on both the unemployment rate and total output."

But despite the feeling that public service jobs are costly and perhaps temporary, this is a principal direction in which the federal government is moving in order to restore health to the economy.

Carter's economists calculate that by providing an extra 750,000 or so public service jobs, the unemployment rate can be cut from 7.8 per cent at the end of 1976 to about 7 per cent or less at the end of 1977, with the rate moving down the 6 per cent mark by the end of 1978.

This assumes that the rest of the Carter economic stimulates package, including a tax rebate, also proves to be effective medicine.

What in fact, are public service jobs and for whom are they created?

A public service job may be defined as once on a local government payroll, financed by Uncle Sam, to fill a low-level responsibility - probably one that would not have been created unless it were being subsidized.

According to William Hewitt, acting assistant secretary of labor for employment and training, it might be for a teacher's helper, a hospital aide, an assistant in a social service center, a clerk in the country jail.

Still other typical public service jobs include a handyman painting public buildings, a park grounds keeper, an all-round helper in a public park.

There is a continuing program of this kind managed by the Labor Department under the Comprehensive Employment and Training Act (CETA) of 1973, which the President Carter proposes to expand from a current ceiling of 310,000 slots to 600,000 by the end of the year, and to 725,000 next year.

The top salary allowable under the program is $10,000, and the average is about $8,300 or $8,400, according to officials.

The new program is directed to the 4 million or more (of the total of 7.5 million unemployed) whom Council Economic Advisers Chairman Charles L. Schultze says would have a hard time finding a job, even in good times.

Of about 300,000 public service jobholders on the present CETA rolls, about 65 per cent are white, and 62 per cent male, according to government data. The largest single group is 22-24. Two out of five are high - school graduates. Minority groups account for a larger percentage - approaching 50 - of special CETA programs concentrating on training.

There have been many studies of CETA programs and its predecessors going back to the Great Society programs of the mid - '60s, but none that claims flatly that public service jobs significantly improve the future earning power of the program's "graduates."

But proponents, like Sar Levitan of the Center for Manpower Policy Studies at George Washington University, claim that there are many noneconomic benefits.

Even if the expanded CETA program is successful, it will provide jobs for perhaps no more than one-tenth of the total number now out of work. But Carter, responding to a caution by Labor Secretary Ray Marshall, says that an additional 750,000 jobs is "an ambitious" goal, and that it would not be productive to try to do more. Congress apparently agrees.

But how about public works jobs? What is the outlook there? A public works job, in contrast to a public service job, is created specifically for workers through government financial of buildings, roads, or other such projects.

Against the $10,000 maximum per job cost in the public service sector, it costs an estimated $25,000 to $40,000 to create one public works job, when one takes into account brick, mortar and other materials, and the salaries of supporting an administrative workers. The $25,000 to $40,000 cost is referred to as the "on-site" cost, because there are clearly "off-site" cost, because there are clearly "off-site" benefits in terms of jobs that may be created in steel mills, brick-making establishments and so on.

Almost everyone agrees that in terms of immediate economic impact, it's quicker to put someone on a payroll (public service employment) than to start up a public works project, where it may take a year or two to hire the first construction worker.

A manpower expert on Capitol Hill said: "We really don't have a good sense of how much an accelerated public works program would be new" compared to what otherwise would have been started.

The AFL-CIO, on the other hand, a strong proponent of public works programs that would benefit its construction trade unions, says there is a backlog of $24 billion worth of projects competing for the $2 billion in money that was authorized last year. It contends that the time gap between allocation of funds and a start on the work, usually cited as a restraint on the program, has been exaggerated.

It contrast to the national unemployment rate of 7.8 per cent in December, 1976, the jobless rate among construction workers was 13.9 per cent. Says AFL-CIO President George Meany: "The obvious conclusion is that highly skilled workers have left the industry in which they spent many years training, in order to take any kind of a job, or they just dropped out of the labor force."

Even with a boost in the public works segment by Congress, there will apparently be a bigger concentration on public service jobs in the stimulus package when it comes out of Congress.

In addition to the increase in what might be called the "regular" CETA jobs to an ultimate total of 725,000 next year, Carter asked for yet another 346,000 jobs to be created for special purposes.

Among special programs would be one to put 176,000 youngsters into a Civilian Conservation Corps along the style of the Depression years' CCC; one for 92,000 Vietnam veterans, and one for 58,000 migrants and Indians.

About $1 billion of Carter's proposed $6.1 billion for manpower programs (other than public works) would be extra money for the "counter-cyclical) revenue-sharing program" passed by Congress in 1976.

Under this program, additional funds are given to states and localities, according to a formula relating to their unemployment problems, when the national jobless rate exceeds 6 per cent.

Carter's program would extend the "counter-cyclical" device through fiscal 1978, and liberalize the money grants. In essence, that would enable the states and cities to keep on payrolls those who would otherwise be fired or add to their own public service equivalents of the federal programs.