KFC plans to bring back the Double Down burger in an effort to revive sluggish revenues.
The chain's owner, Restaurant Brands, which also runs the Starbucks and Pizza Hut chains, reported a fall in sales in the 12 weeks to the middle of May.
Sales at stores open more than a year fell 4% to $68 million in the May quarter, compared with the same period a year ago.
KFC is Restaurant Brand's main earner. But its sales fell by more than 6%, though the high-profile and successful promotion of its Double Down burger boosted sales last year.
The burger - which consisted of bacon and cheese sandwiched between fried chicken fillets instead of bread - alarmed heath advocates, but proved a hit with customers.
Restaurant Brands plans to put the burger back on its menu for a limited time later this year, in an effort to boost revenue.
Starbucks coffee sales slipped 1%, while Pizza Hut stores reported 10% growth in same store sales, due to promotional activity for its cheaper pizzas.
Restaurant Brands still plans to open its first Carl's Jr burger store later this year, and says it has several other sites under consideration.
Shares in Restaurant Brands were down 4 cents to $2.07 on Wednesday.