One of the reasons people make carbs fat on the bottom is that we are asking the carb to do something they normally aren't designed to do.
We are asking the carb to make as much HP as possible on the bottom end so it comes off the corner with out hesitation.
The conventional concept of a carb will usually be said to have the following circuits:
Idle circuit -- Just for idling
Transfer circuit
Mid range
High speed circuit -- for WOT
You might also add an acceleration circuit
In racing for the most part we don't care about idle. We only care about coming off the corner as hard as possible and max HP at WOT.
MOST of the carbs we use for kart racing do not have an accelerator pump circuit so people make the idle circuit overly rich to compensate for it.
Imagine flying down the straightaway and then lifting when you go into the corner.
When you lift the throttle plate or slide closes and you get a very high vacuum (close to 30 inches of HG) in the entire intake tract in front of the throttle plate or slide.
The idle circuit and transfer circuits are both subjected to this extreme high vacuum and fuel pours out of those circuits into the entire intake and even the exhaust tract. That includes the the carb in front of the throttle plate, the intake manifold, the cylinder and the exhaust system.
Talk about washing the oil from the cylinder walls from the excessive fuel (gas or more so methanol)
On a methanol engine a lot of this excessive methanol ends up in the crank case / oil. Change it often!!
On decel you have this massive amount of fuel throughout the entire upper end of the engine just waiting for some Oxygen so it can burn.
If everything is right when you get back on the throttle you feel yourself up against the back of the seat and everything is right again in the world.
The rush you get when that dopamine is released is way better than booze or drugs!
The down side of an overly rich idle circuit is that the engine either doesn't idle or idles very poorly. This overly rich idle circuit can also dilute the oil and foul spark plugs (especially on gas)
Some people build carbs with the idle circuit richer than it needs to be.
It can take a long time on the dyno to get the idle circuit big enough to have max acceleration with out overdoing it.
In this tuning process then you have to make other circuits a little bigger to compensate. Correction: in this tuning process you have to make the other circuits the correct size to compensate for the size you made the idle circuit.
I would say there have been a lot of karting engines blow up because of too much fuel in the oil and or not changing the oil frequently enough.
Idling during a long caution and continually punching the throttle ain't good!!