Motorcycle accident deaths jumped 46 percent after states began repealing laws requiring cyclists to wear helmets, the National Highway Traffic Safety Administration reported yesterday.

Head injuries caused by motorcycle accidents doubled and tripled in the 27 states that took off their helmet laws, a study by the traffic safety agency said.

The states began repealing the laws after Congress in 1976 backed down on federal threats to force states to require helmet use. The lawmakers repealed a provision that let the Department of Transportation cut federal highway construction funds by 10 percent to states that did not have a helmet law.

The federal threat had lead every state but California to require motorcyclists to use helmets. When the helmet laws went into effect in the late sixties, motorcycle death rates dropped 40 percent.

"Many states have interpreted the repeal of our authority to impose sanctions as a signal to revoke their helmet laws," said NHTSA administrator Joan Claybrook.

Congress ordered a full-scale study of motorcycle accidents after deaths appeared to take a dramatic jump in states that dropped their helmet laws.

The report by NHTSA said motorcycle fatalities increased from 3,312 in 1976 to 4,850 in 1979. The study reported a two- to three-fold increase in the frequency of head injury for states which have repealed their helmet laws and almost a four-fold increase in the severity of head injuries.

The safety study also challenged the claims of motorcyclists that helmets cause accidents and make neck injuries worse.

"It was found that helmets do not cause neck injuries," the NHTSA concluded. "Documentation of whether helmets cause accidents by impairing vision was studied and found not to support the claim that helmets harm vision."

Federal researchers found at least 95 percent of motorcyclists use helmets in states that make them mandatory, but only 50 to 60 percent wear them where use is voluntary.

The District of Columbia and Virginia are among the 22 jurisdictions that still require helmets. Maryland makes helmets mandatory only for riders under 18.