A gas engine in the 50cc-60cc range can run more than 20 minutes on a 24oz tank. I have 32oz tanks in my 100cc twin cylinder giants and use barely half of it in 15 minutes of IMAC sequences. I have 24 oz tanks in my 50cc planes and same thing. My 170cc twin takes a bit more though(64oz tank).
Just sayin'. Carrying an extra 8oz's or more of fuel that will never be used is quite heavy.