Eastern senators yesterday threatened a filibuster to defend a proposed federal regulation that would cut fuel oil and power costs along the East Coast at the expense of the rest of the country.

The senators warned that they would try to talk to death an amendment that would block implementation of a plan put forth by the Carter administration to triple the entitlement, or subsidies, paid to eastern refiners and utilities that import heavy fuel oil.

The threat came after a full day's negotiations between a dozen eastern senators and Sen. J. Bennett Johnston (D-La.). Johnston has sponsored legislation to block the subsidy increase for the East.

Johnston was outraged this spring when the Energy Department first revealed its plan to increase subsidies for oil importers in the East. He said the change could only encourage greater imports, setting back the national effort to reduce dependence on foreign energy supplies.

He quickly introduced legislation to block the subsidy increase in its entirety. That bill passed easily in the Senate Energy Committee, but for various reasons it has not yet come before the full Senate.

Thus Johnston decided to bring the bill to the floor yesterday as an amendment to an Interior Department appropriations bill. That move prompted the day-long bargaining session pitting eastern senators against Johnston.

Johnston told the easterners that he had the votes to pass his amendment cutting off the subsidy increase. The easterners told Johnston that under the Senate rules they could filibuster, preventing Johnston's bill form ever coming to a vote.

The filibuster threat alarmed Majority Leader Robert C. Byrd (D-W. Va), the Senate's schedule maker, who hopes to finish work on several money bills this week. Byrd pressured Johnston to compromise to avoid a talkathon.

Accordingly, the two sides hammered out a deal in which the East would receive some increase in subsidies, but less than the administration proposed and for a limited time period.

By 4 p.m. yesterday the two sides were in general agreement on the compromise plan, but proceedings hit a snag when Energy Secretary James R. Schlesinger Jr. balked at the proposal.

Schlesinger was concerned because the senators' compromise involved a waiver of an import tariff levied against eastern refiners and utilities using imported fuel. Import fees could be an important presidential tool for reducing dependence on foreign sources, and Schlesinger was reluctant to negotiate that tool away.

The "entitlements" program is a complicated pricing system designed to even disparities created by the government's oil pricing rules.

Under federal regulations, domestic oil is considerably cheaper than imported oil. Since the East, and particularly New England, uses more imported oil than other regions, eastern consumers have been paying more for oil than consumrs elsewhere.

The "entitlements" rules force refiners in the western two-thirds nation to pay subsidies ("entitlement") to refiners, utilities and industries in the East to offset the higher price of imported fuel.