If the founding fathers were not liberals...what would you call the Torries who were loyal to the King?
Liberalism relates to the desire to look at things as they are and evolve them to a changing environment. Conservatives seek to retain the current structure. Neither position remains static over time.
Conservatives of today are called so because they wish to retain the structure of government as it was when it was first formed, meaning after the revolution and writing of the Constitution. So to claim that conservatives of today would of been loyalists back then is absurd.
Liberalism in the 18th century America isn't anywhere near liberalism/progressivism of today. Liberals of today are seeking to change America from what it was founded as, to a secular socialist nation.