SEATTLE (Reuters) - Warning of an economic "tailspin," Starbucks Corp (SBUX.O: Quote, Profile, Research) outlined long-awaited plans to turn around its U.S. business on Wednesday, but details from new coffee machines to a rewards program for frequent customers failed to excite investors, who sent shares down 4 percent.

Chief Executive Howard Schultz told the company's annual meeting there was no "silver bullet" for fixing Starbucks, whose stock has dropped 40 percent over the last 12 months…