I'm not sure I buy the "product of their time" argument on this one. That works when you're asking why Franklin wasn't considering quantum mechanics in his electrical experiments, or why Washington's doctors didn't give him antibiotics when treating him for the fever that killed him. These things were beyond their ability to manufacture or even perceive at this time. This is not the case for slavery.

There were plenty of abolitionists active during the American Revolution, and VERY few people would have viewed slavery as a moral good. Certainly, even slaveholding Founding Fathers (hyperbolicly) used slavery to describe what the Parliament was trying to impose on the colonies. Jefferson famously described slavery as holding a wolf by the ears: no one is happy to be there, but there is no easy or bloodless way out of the situation. Which is to say, many of the Founding Fathers knew slavery was wrong, and did it anyway because it was easy and profitable.

The book you probably most want to check out is Alan Taylor, The Internal Enemy. It looks at how white Virginians, notably Washington, Jefferson, and Patrick Henry, viewed slavery from the American Revolution to the War of 1812. In brief, slavery was key to their sense of wealth and property as it currently existed, and they consciously refused to consider other labor systems. They also saw African Americans as a growing security concern, as they could only picture ex-slaves taking bloody vengeance against their former masters. As such, while some, like Washington, freed their slaves in their wills, they generally supported increasing restrictions on African Americans in life.

It really depends on what you mean by contributing to racism. Are you looking for specific things they did which advanced white supremacy in the United States as contributory?

Or are you more asking about specific racial doctrines that they might have invented?