What were the Founding Fathers' views on slavery?

This is a very broad question that we can answer best by examining the views of four men whom I would consider the most important of all of the Founders: George Washington, Thomas Jefferson, Benjamin Franklin, and John Adams.
Though Washington and Jefferson were practitioners of slavery, both of them knew slavery was wrong. Like most of the more intelligent and sensitive Southerners of their generation, they felt trapped by a system they had inherited. They also believed that they could not abolish that system outright because 1) there was not enough support among the Southern planter class to make abolition successful and 2) abolition would be impossible while the other events of the time, and the sweeping changes and upheavals caused by them, were occurring. From 1754 to the end of the century, America went through the French and Indian War (1754-63), the War of Independence (1775-83), the Constitutional Settlement (1787-88), and the first presidential administration of the newly independent country (1789-97). In the view of Washington, Jefferson, other Southerners, and even those among the Founders who were totally opposed to the institution of slavery and had no personal stake in it, abolition would produce conditions endangering the survival of the colonies, the success of the independence effort, and the fragile safety of the newly-established United States. It was at a time when the new country could easily have come apart or been plunged into chaos, as Britain probably hoped it would and even sympathetic Europeans thought it might. This is not to defend Washington, Jefferson, and others for essentially putting the slavery issue on the back burner as they did, but these factors were what formed their rationale for doing so.
Washington refused to buy or sell any enslaved persons from 1775 (the start of the War of Independence) on, and he freed all of his slaves in his will. The same cannot be said for Jefferson. Despite having written the eloquent words of the Declaration of Independence, Jefferson freed only a handful of slaves and resisted all suggestions to enact even gradual emancipation plans for Virginia or the original states of the Union as a whole. He was in favor, however, of banning slavery in the territories and pushed into reality the Northwest Ordinance of 1787, which established the ban. A ban on slavery in the remaining territories south of the Ohio River, which Jefferson also supported, was narrowly defeated.
The other two Founders we are looking at, Franklin and Adams, were unequivocally against slavery. However, they both at various times, in the interest of what to them was the greater significance of independence from Britain and the formation of "a more perfect union" in the Constitutional settlement, did not press for the issue of abolition in 1776 or 1787. In 1776, both Adams and Franklin could have spearheaded an effort to include a mention of slavery in the Declaration, but they did not. In 1787, Franklin knew that to make slavery an issue at the Constitutional Convention would cause the Southern delegates to walk out. In summary, we must say that Washington, Jefferson, Franklin, and Adams all believed that slavery was wrong but subordinated the issue to other factors in the hope that someday, when the new country was stable and unthreatened by forces internal and external, the institution would be abolished or "die a natural death." Emancipation finally was enforced by the Union victory in 1865, 75 years after Franklin's death, 66 years after Washington's, and 39 years after the deaths of Jefferson and Adams on the Fourth of July in 1826.