LONDON, April 3 (Reuters) - Africa's top copper producer Zambia is talking to investors this week about a possible second dollar bond, but investors say its days of achieving sub-6 percent yields are past and that it may have to pay more than 8 percent.

Zambia's was one of the first in a wave of African dollar bonds to launch in recent years, and one of the best received, but it has underperformed other sub-Saharan African Eurobonds in recent months.

That reflects the impact of U.S. stimulus withdrawal, which has pushed up yields across emerging markets, and a tumble in the price of copper as the economic outlook darkens for giant resources importer China.

Zambia's kwacha currency ZMW= has also hit record lows.

A roadshow for the new bond reaches its final leg in London on Thursday and Friday, according to Thomson Reuters news service IFR.

The yield for a new 10-year dollar bond should be at least 8 percent, said Stuart Culverhouse, chief economist at frontier markets broker Exotix.

"It's more expensive than Zambia paid two years ago - maybe they should go a bit higher than 8 percent, given uncertainties about the global outlook and copper prices."

Copper CMCU3 has dropped 20 percent in price to $6,700 a tonne, from $8,400 in Sept 2012.

Zambia's debut $750 million 10-year bond was oversubscribed by a spectacular 15 times when issued 18 months ago at the height of investor excitement about frontier market debt.

It benefited from investor demand for higher-yielding debt as the Federal Reserve's quantitative easing programme - which it has now started to wind down - depressed U.S. yields.

The yield on Zambia's existing bond has risen by more than 200 basis points to 7.8 percent XS082877959=TE , according to Tradeweb, and has underperformed similar debt, the following graphic shows: http://link.reuters.com/set57v

New bonds usually command a premium to encourage switching from existing debt.

"I would presume they would want to do a 10-year, a good starting point would be 8.25-8.50 percent," said Richard Segal, analyst at Jefferies.

But a juicy yield will still bring demand for the B/B-plus rated credit, said Angus Halkett, emerging debt fund manager at Stone Harbor.

"Zambia has widened quite a lot, you could argue it was too tight to begin with. There has to be a price at which people will come in and buy."

(Reporting by Carolyn Cohn; Editing by Catherine Evans)

((carolyn.cohn@reuters.com)(44 207 542 6320)(Reuters Messaging: carolyn.cohn.thomsonreuters.com@reuters.net))

Keywords: ZAMBIA DEBT