Jimmy Fallon -- I Asked Howard Stern for Advice Before Obama Interview

Before Jimmy Fallon's big interview with Barack Obama last month, the talk show host reached out to several people for advice ... including the King of All Media himself ... Howard Stern.

Stern appeared on Fallon's late night show last night ... and admitted he almost ignored Fallon's plea ... because he's "somewhat evil" and couldn't figure out what was in it for him.

But the "America's Got Talent" judge ultimately decided to help ... and says he encouraged Fallon to ask Obama about the following subjects:

-- How big is your penis, Mr. President?
-- How often do you have sex with Michelle?

Unfortunately, Fallon didn't take Stern's advice.

But Jimmy DID manage to piss off Howard when he revealed who else he called on his quest for interview advice ... including Jay Leno.

Stern shot back ... "I never would have answered your email if I knew you called Jay."