For the past two nights, some 100 fuel-starved motorists have packed into the Washington Hilton at $15 a head to learn how to get 200 miles per gallon from the family car.

The secret, they hear from a leisure-suited pitchman named Don Novak, 34, is to heat the gasoline to 150 degrees and run on the fumes.

Of course, added the unemployed Wichita aircraft engineer and apartment maintenance man, they might be advised to purchse a reliable fire extinguisher and an extra life insurance policy - just in case the car explodes.

"Gasoline can be real forgiving, but if you miss one time, you won't live to tell about it," said Novak, one gray polyester figure beneath crystal chandeliers.

Since the beginning of time, every drought has spawned a medicine man who boasted he could make it rain. Now comes the gas drought of '79 and rainmaker Don Novak, the first of a probable torrent of fervent, how-to inventors blowing into town on the winds of the crunch.

"I don't know he's talking about," mumbled Tom Eldridge, 30, a baffled management consultant from Arlington who approached Novak during a break to ask what he was talking about.

"I'm lost and i'm not alone."

Eldridge, who admitted to being all thumbs beneath the hood, had been making faces of anguish.

"I though you had an attitude problem, but I'm glad to see you're really interested in learning," said Novak.

He pointed toward a make-shift air filter stuffed with styrofoam and copper Chore Girl scouring pads, the secret weapon that he feels could make him a folk hero and bring the oil giants, sniveling, to their knees.

It has not been perfected yet, he said, but the styrofoam is supposed to soak up gas poured into the chamber. The Chore Girls are thrown in to prevent a bad fire.

Under the best conditions, the gas vaporizes feeding fumes into the hungry carburetor. Meanwhile, the fuel line has been rerouted through the radiator to heat the gas to 150 degrees, adding further efficiency.

On a hot day, however, improperly vented vapors could accumulate and "blow the hood off the car," winked Novak. "But that won't really hurt you."

One student, he claimed, is cruising 230 miles on a gallon of gas in a Novak-inspired Chevy Blazer. For $10, he will entertain callers and forward diagrams.

Half the predominantly blue collar crowd of mechanics, truck drivers and construction men who did not raise their hands to admit confusion hunched over yellow legal pads and scribbled down every word.

"What we're here for is to get back at the oil companies, keep them from ripping us off," groused Bill Hardesty, 57, a burly ironworker whose wife, desperate from waiting in long gas lines, had head Novak's rambling delivery on a radio talk show several weeks back.

Aluminum siding salesman Jim MacDonald, 54, of Annandale, a former broadcaster, tuned in the same show and spied an opportunity to set up local seminars for Novak at $250 a shot.

Paul Brown one DOE official who chewed over notions with Novak, said the government keeps an open mind for energy-saving innovation and, in some cases, will even pay cold, hard cash to anyone with a better idea.

Novak's ideas were nothing new, said MacDonald, simply a compilation of the ideas of others, presented in droning disjointed fashion.

EPA officials said such devices might violate antitampering laws regulating emission control devices, not to mention the risks of immolation.

"Sure, people think it's a hoax," said an undaunted Novak, a man with a physics degree who has laughed off being laughed at before.

"But this carburetor project could be worth $100 billion. It would cut gas consumption 900 percent. "I get millionaires coming up to me all the time saying, 'If you can prove it works, I'll give you a million . . . to get it off the ground',"

He hasn't gotten around to it yet. "Engineering is just a hobby to me," he says.

Novak was an unemployed engineer with a checkered career until January, when the gas crunch hit and his lectures at an open university in Kansas began outdrawing the barhopping class.

He's been barnstorming the Midwest ever since, hawking his "Hundred Mile Per Gallon Seminar" from town to town, waiting for the oil companies to bump him off. "I never thought I'd get to Washington alive," he says.

But energy pioneers have to take a few risks, he says, even if it means filling the old air filter with unleaded and cruising off in a 200-mile-per-gallon time-bomb.

"People all over the country are doing this," he says. "It's just plain old mechanics." CAPTION: Picture, Don Novak displays components of his super-efficient gasoline burner.