i am learning crankflips and i am just getting the zero plant but every time i do it i flip the cranks to fast and almost do doubles… when i try to do it gently they dont even spin… any help on how to get the right timing would be great 
you’re probably not kicking the cranks. there’s a good chance that what your doing is riding along, and instead of jumping up and kicking the cranks before putting your foot down you, you lean back and let the wheel roll out from under you, thus resulting in it spinning as fast if not faster than you were riding. the when you try to kick them gently you ride slower thus resulting in the wheel spinning much slower. i had the same issue.
i don’t know if you’ve read the other crankflip threads, but it’s best to learn to kick the cranks before learning the zero plant. what you’re gonna want to do is ride along at a decent speed. when your cranks reach your desired hopping position lean forward and put all of your weight onto your front foot. while still keeping your weight forward jump straight up; that’s one of the key factors you have to jump UP not OUT. when you get into the air as long as you’ve kept your weight forward you will kick the cranks and make them spin as your leaving the ground. once in the air don’t worry about landing it, just bail and then do it again.
once you’ve gotten good at flipping the cranks then go back and try to zero plant, but be sure to apply the same steps except this time instead of bailing perform a zero plant.
hope this helps