No need to mix with gas if you're using the cleaner to dip your carbs in. Mixing it with gas is for when you just fill the float bowls while the carbs are on the bike(directions on back of bottle), and it dilutes the cleaning power. No harm in doing it your way, just don't go lighting any cigarettes in the garage
Does your ma like to bake bread? Maybe she's got an old bread pan that would juuuust fit the carbs. Trick is to find something rectangular and only slightly larger than the carbs so that less cleaner is needed to fill the space. If you've taken off the bowls and removed the jets and can only submerge the carbs halfway then make sure it's the bottom half and that the cleaner level comes at least 1/4" above the bottom of the throttle valves. All the passages that need cleaning are in the bottom half of the carbs. You may want to turn the carbs over in the morning when you see how clean "half" your carbs are
That Yamaha cleaner ain't cheap so I'd see if you can make do with what you've got.
