Lemon juice is perfectly fine for cleaning carbs if you are sensible about it.
The EPA over the last several years has been cracking down on
harsh chemicals like carburetor and parts tank cleaners.
With this crack down, almost all parts tank cleaners are a citrus based
cleaner that is comprised of orange and lemon juice.
While not as effective as the good old carb cleaner, it will work and it
wont' hurt anything. Obviously, don't throw it in and leave it sit
for weeks on end, but an over-night soak won't hurt a thing.
If you use it, I would suggest afterwards, washing it down with a good
base of some sort to make sure all cleaning (acidic) action has stopped.
Gasoline is for washing parts, Alcohol is for drinking... NITRO is for racing!