I know there are pros and cons to most things, but I don't like using slime or fix a flat for several reasons.
1) Even with aluminum rims, it will corrode the inside of the rims over time.
2) If you live where it gets cold in the winter, slime will freeze in a puddle in the tire and vibrate your fillings loose.
3) As mentioned before, your tire guy will never change another ATV tire for you after he and his tools are covered in snot. It makes a nasty mess.
You can buy a 30 pack of plugs for what a small bottle of slime costs. You can also buy an elcheapo foot pump for the trail to take with you.
The easiest way to find the leak is to overinflate your tire, and then take a spray bottle full of soapy water, or you can even use Windex - then spray the tire until you see a steady stream of bubbles coming from the tire. Now you found the leak. Plug it, set the correct pressure, and go ride.
