LUSAKA, ZAMBIA -- Amos Lukumba's tool of economic analysis is a hammer.

Lukumba, who drives a taxi, developed his hammer-based school of economics after noting a disturbing trend of import substitution in this landlocked country. Unable to afford foreign imports, a select number of the cabby's countrymen have sought out his cab in the dead of night and swiped his windshield. Four of Lukumba's windshields have disappeared in the past year.

To check this trend, the cabby took his hammer and bludgeoned his present windshield, causing a cluster of cracks that would turn to shards if his windshield were removed. In the Zambian context, Lukumba believes it good business to destroy his windshield in order to save it.

The president of this debt-hobbled southern African country has recently embraced a similar economic philosophy. Kenneth Kaunda last month swung a rhetorical hammer and destroyed an economic recovery plan that guaranteed Zambia hundreds of millions of dollars in foreign loans and grants.

Kaunda did it, he said, because the conditions attached to the money from the International Monetary Fund, the World Bank and other western donors were intolerable. Debt repayment, he said, would finish his country. He was destroying Zambia's economic recovery plan in order to save Zambia.

Kaunda's hammer blow has rattled the foundations of a free-market reform movement that has swept across sub-Saharan Africa in the past three years. While the reforms have revived the economies of two dozen poor countries, they also have saddled them with unpayable debts.

Zambia, in effect, made a unilateral declaration of national bankruptcy and thus forced western donors to acknowledge there is something fundamentally wrong with the way they are trying to salvage African economies.

For example, the total export earnings of the country are forecast to be about $650 million this year. With this, Zambia was supposed to pay debts of about $400 million to the IMF, about $71 million to the World Bank and nearly all the rest to various donor countries. That would leave nothing to cushion the hardships of the most ambitious economic reform package in the country's history.

If these debts are not paid or rescheduled, using high-interest "bridging" loans through commercial banks, IMF and World Bank regulations require that Zambia be barred from further assistance.

Kaunda, in his dramatic speech on May 1, said the debt burden would undo any positive effects of reform and bring Zambia to "an economic and political halt."

Western economists here who presided over the creation of the reform package agree with Kaunda that the foreign debt burden is, as one economist put it, "killing the country."

"The international community should have been aware that the debt situation was hopeless," said Sunit Ray, an economist with the Swedish government. "I don't think the IMF made sufficient preliminary studies. It did not do its homework."

Western economists here characterize recent large IMF loans, most of which are supposed to be paid back in full within three years, as "criminal," "illogical" and "a mission impossible from the start."

The IMF recently conceded that it made serious miscalculations in determining Zambia's ability to repay the loans. In a conference this month in Nigeria, the director of the fund's African department said the IMF erred by anticipating unrealistically high prices for copper, Zambia's major export.

"To continue to pretend that we don't know that countries like Zambia cannot pay back the money is criminal," said a senior western economist who has been closely involved in Zambia's economic planning. "You can't keep on lending money to somebody who is bankrupt and expect them to pay."

This message, because of Kaunda's resounding demolition of a reform package that the World Bank last year cited as a model in Africa, appears to be shaking policy makers in Washington.

According to a World Bank official there, meetings have been held at the bank and IMF to consider exceptions in scheduled loan repayments by poor African countries.

The World Bank says that there are at least 11 other "debt-distressed" African countries that appear unable to repay their debts.

The major objection to changing the rules and possibly forgiving the debts is raised by the U.S. government, which has a key voice in IMF policy. The Reagan administration maintains that such a precedent would encourage all debtor countries to follow Zambia's lead.

Several western economists here insist that concern about setting a dangerous precedent must be tempered by realism.

Africa, the world's poorest continent, squeezed on one side by falling commodity prices and on the other by mounting debts, is in a different category from the rest of the world, these economists say.

"This is not a precedent for not paying back money," said one economist who represents a major western creditor. "Rather, it is an admission that lenders like the IMF have badly misjudged the credit worthiness of countries like Zambia. If the people who made these loans worked for private banks, they would be fired."

While the West ponders what to do about Zambia's challenge, there is little doubt here that this country, despite its brave rhetoric, is headed for economic and political upheaval.

The World Bank has halted spending on most of the 24 long-term projects it finances here. The cost to Zambia, if the money never arrives, could be as much as $400 million. In addition, about $160 million, which was to supplement the country's shortage of foreign exchange for essential imports and spare parts, is now lost.

More fundamentally, Zambia's recently announced go-it-alone economic plan is a return to the policies that sabotaged the country's enormous farming potential for most of the past 20 years.

There is little question that the IMF reforms, by raising prices paid to farmers and by giving businessmen access to foreign exchange to buy spare parts, were working.

In 1986, for the first time in five years, farmers grew a surplus of corn. Availability of foreign exchange to buy spare parts boosted factory use from 20 percent of capacity in 1985 to 50 percent in 1986.

"If this system had been given time, . . . it would have worked," said economist Ray. "These reforms are the only thing that could have saved this country."

The long economic fizzle that preceded Zambia's short and miserable marriage with the IMF is a depressingly familiar African story.

Like most African countries, Zambia has been dependent on commodity earnings. Here, it is one commodity, copper. Before 1974, huge reserves and high copper prices made Zambia rich. Kaunda, the country's founding father who is committed to a socialistic system of distributing the nation's wealth, used copper money to build hospitals, roads and schools.

As people rushed to cities, Zambia became the most urbanized country in black Africa, with nearly half its people living in towns. The country lost its ability to feed itself, but food was imported and copper paid the bills.

Since 1974, however, copper prices have fallen by two-thirds. The government has been faced with two choices, either to shrink the standard of living of its people by two-thirds or to borrow money. Hoping that copper prices would rebound, it borrowed about $5 billion. Now, with copper prices likely to stay down, those debts are thought to be unpayable.

So far, however, there appears to be little willingness by Zambians to adjust their standard of living accordingly. The elimination last December of price subsidies on cornmeal, which doubled its cost, caused rioting that killed at least 15 people. The government backed down, reversed the price increase and reinstated a subsidy it cannot afford.

"It is always more difficult to go down in your standard of living, and this IMF program is a drastic step down," said Ray.

But western economists and diplomats from donor countries say that Kaunda's denunciation of the IMF and his plan to return to a state-controlled economy will quickly force an even more drastic step down.

New price controls on essential goods like soap and cooking oil have, in less than two months, led to the disappearance of these items from stores. They are available only in the booming black market, and there prices have doubled. Several factories, such as the large Dunlop tire plant, have closed in recent weeks for lack of foreign exchange to buy raw materials.

There is a remarkable consensus among advocates of capitalist change here that the IMF, by putting unrealistic conditions on loans, helped submarine this country.

"The IMF medicine was correct," said Ray. "But the dosage was too strong. The time frame was too short."