China's economic weakness and yuan devaluation aren't good news for the many U.S. companies that have dreams of riches there.

China reported economic growth of 7 percent for the second quarter, but economists say that number is vastly overstated. Some say the true figure is 3 to 4 percent. A weakening economy, of course, limits sales opportunities for U.S. companies in China.

Then there is the yuan, which has slid 3 percent since China devalued it last week. A falling yuan hampers U.S. exports to China by making them more expensive in yuan terms. And it lessens the value of U.S. companies' revenue in China when that revenue is converted into dollars.

To be sure, China isn't a big deal yet for most major U.S. companies, writes Theo Francis of The Wall Street Journal. Only 16 S&P 500 companies garner at least 10 percent of their revenue in China, according to Wells Fargo Securities.

"But the promise of China’s vast population and rapidly expanding middle class, along with a gradually liberalizing business climate, have lured big companies despite the risks," Francis explains.

Many major U.S. companies have cited negative effects from China in their second-quarter earnings conference calls.

The yuan devaluation may hurt more than U.S. companies, says former Treasury Secretary Larry Summers.

“The risks of a deflationary, secular stagnation in the U.S. would be increased by a large devaluation of the renminbi,” the Harvard professor, told The New York Times.

The move is deflationary because a weakening renminbi makes U.S. imports of Chinese goods cheaper in dollar terms. And it pressures U.S. exporters to cut their prices in China, because their goods are now more expensive in yuan terms.

To be sure, it's not clear how serious the situation is yet, Summers said. “One has to be very careful about regarding market fluctuations and uncertainty among market participants as a crisis that demands major government interventions.”

The dollar traded at 6.40 yuan Monday, up from 6.2086 yuan Monday.

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