US STOCKS-Wall St to open flat, investors eye Washington talks

* AutoZone (NYSE: AZO - news) edges up, results show profit growth

* Carnival (LSE: CCL.L - news) slides, hurt by downgrades, forecast of loss

* Futures: Dow up 16 pts, S&P up 0.3 pt, Nasdaq up 4 pts

By Rodrigo Campos

NEW YORK, Sept 25 (Reuters) - U.S. stocks were set to edge up at the open on Wednesday, following four days of losses on the S&P 500 and Dow, even as a lack of progress in budget negotiations in Washington left investors puzzled.

Concerns over a potential U.S. government shutdown added to recent mixed signals on the immediate future of the monetary policy that has given support to equities, keeping traders on edge. Still, the possibility of a last-minute deal in Congress eased the selling pressure brought by the uncertainty.

"Investors are not willing to sell the market only to see some kind of agreement reached that would give it support, nor jump in knowing it could get a lot uglier before it is resolved," said Rick Meckler, president of investment firm LibertyView Capital Management in Jersey City, New Jersey.

He said there was confidence an agreement would eventually be reached, but "it seems we can't get out of the way of the political games that surround funding and operating the government."

Most Republicans shunned Texan Senator Ted Cruz's marathon diatribe against President Barack Obama's health insurance reform law, which was delaying Senate consideration of a stop-gap funding measure needed to avoid a government shutdown in six days.

S&P 500 futures were flat and slightly higher in terms of fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones (DJI: ^DJI - news) industrial average futures rose 16 points, and Nasdaq 100 futures added 4 points.

Orders for long-lasting U.S. manufactured goods edged higher in August and gave a signal that the factory sector gained a step, midway through the third quarter. New home sales data for August is due at 10:00 a.m. (1400 GMT).

AutoZone shares edged up 0.7 percent in light premarket trading after the largest U.S. auto parts retailer reported a 15 percent rise in quarterly profit, mainly due to higher margins.

Medical device maker Stryker Corp will buy smaller peer Mako Surgical (NasdaqGS: MAKO - news) for about $1.65 billion to gain access to Mako's technology for robot-assisted orthopedic surgery. Mako shares surged 83 percent in premarket trading.

Carnival Corp shares fell 4.6 percent in premarket trading after several brokerage downgrades in the wake of the company's warning it could report an adjusted loss for the current quarter.