President Carter's new energy measures may help slightly to hasten the economy's recovery from recession next year, but they also could add to inflation over the longer run, according to key economists.

Analysts say Carter's plan to spur more government and private spending on synthetic fuels production and converting oil-fired utility boilers to coal could begin to quicken the economy's pace as early as 1980.

However, economists caution that the program also could heighten inflationary pressures later, both by straining the nation's construction capacity and by siphoning investment monies from other ventures.

Some experts question whether the nation can build 30 synthetic fuel plants over the next 10 years - as would be required to meet Carter's 2-million-barrel-a-day goal by 1990 - with the present construction force and equipment.

As Carter described his new plan, the measures would have virtually no impact on the economy or the budget this year and only a modest effect in 1980. The major outlays won't come until 1985 or later.

White House officials said yesterday the administration already has included the effects of the new proposal in the mid-year economic and budget forecasts that it published last week.

And Carter said virtually the entire $142 billion in spending and new federal loan guarantees he expects to propose for between now and 1990 would be financed by revenues from his pending "windfall profits" tax plan.

However, Carter yesterday hedged on both those points. In a speech to the Communications Workers of America, he pledged to seek a tax cut - in the form of a rollback of Social Security taxes - if the jobless rate gets "too high."

And the White House said yesterday that if the "windfall" tax does not provide enough revenues to finance Carter's new energy proposals, "downward adjustments to programs will have to be made."

Carter has said he plans to spend $142 billion on his new energy proposals between now and 1990, including a massive $88 billion for synthetic fuels production, $16.5 billion for mass transit and $3.5 billion on solar energy.

Over the long run, that still is $4 billion less than he calculates his "windfall profits" tax would bring in, even at current crude oil prices of $22 a barrel. If oil prices rise further - as they will - the tax take would rise.

There also is the question of whether the government simply will allocate the right to import oil under Carter's new quotas or auction off portions, as has been done in previous years.

If the administration decides to auction - and Energy Secretary Schle-singer is to tackle this question over the next few weeks - it could produce billions of additional dollars in new revenues for the government.

A good deal of the potential impact of Carter's new proposals depends on two factors:

How sharply his new import quota level of 8.2 million barrels of oil a day "bites" into domestic supplies.

How much of Carter's new synthetic fuels program will involve full government financing of these new projects and how much will be limited to mere loan guarentees, in which Washington simply provides backing for private loans.

Lawrence Klein, the Wharton School economist who served as Carter's adviser during the 1976 campaign, says the extra spending could prove "far better" than a tax cut in helping to spur the economy next year, and prove less inflationary.

However, Otto Eckstein, the former Johnson administration economist who now heads Data Resources Inc., cautions that the increase in federal borrowing later could prove inflationary in the longer run - particularly for direct federal spending.

And Alan Greenspan, former President Ford's chief economic adviser, argues the impact could be the same even if the money were for loan guarantees.

Analysts cautioned yesterday that Carter had not made enough details of his new plan public for them to be fully certain about the impact of the new proposals.