Racing in the wet raises the challenge to the driver, team, and equipment.  The most obvious changes are in the tire-track adhesion, with countless unexpected effects ranging from wet stopwatches to melted rain pants on shifter exhaust pipes.  An entire book can be written on the subject, but for the purposes of this article we will focus on the basics.  If you're 75% prepared for rain, you're twice as well prepared as the average team.

Simple Traction Physics

In the case where a track is semi-wet, light misty rain with puddles, yet somewhat dry in one or two fast turns, these tires can be managed to last or abused and worn down in only a dozen laps or so.  Sliding on dry pavement raises surface temps pretty quickly leading to chunking the edges of the tread.  Square edges are beneficial as they are far more effective to channel water away from the contact patch.  "Chunked tires" are slower then squared tires so it pays to be smooth on dry sections over the course of a race.

On wet turns, the dry racing line may have a coat of rubber and tire residue that affects the surface tension or meniscus of water.  In simple terms, it may be faster to stay a little off the dry racing line, and with each track different only driving on it will give you the right answer.  The point is, the dry line may not give the best traction.  On the other hand in a downpour, sometimes running in the tracks of the preceding car will give you the most traction.  It takes snap judgment to be fast in the wet depending on conditions.

Watch the car in front of you and see where they go.  If the track is semi dry try a little higher line off the dry line by maybe a foot or so.  You can tell if it feels better especially on exit.  A little wheel spin on a wet exit will lose speed off the turn but will not chunk the tires.

Wet pavement also results in a lower coefficient of friction (CoF).   Lower CoF provides less distortion forces to the tires.  Most rain tires carcasses are designed for lower forces to achieve the optimal Slip Angle, but to balance that out generally takes dialing in more Ackerman.  More about that in Front-end setup.

Tire pressure factors for rains are different then slicks for several reasons.  Lower distortion forces, temperature and tread compound characteristics are sensitive to how much water is on the track.  Normally we start at about 20PSI front, and 16 PSI rear for full course wet, and drop them depending on how dry it is or will be.  If a race is full wet at first, and expected to be dry on fast turns by the halfway point, we drop the pressures by 5-6 PSI.  Just like with slicks, it increases the contact patch and distributes heat better.

Even with front brakes, the front tires are still under much less stress and heating then the rears.  One of the first things you notice when driving on a wet track is massive understeer.  This is caused by the inside rear wheel not being able to lift, therefore, both rear wheels are pushing the kart in a strait line.  By toeing the front end out slightly helps both with Ackerman and adding some heat down the straights.  It does compromise straight line speed but only slightly.  Consider that down a  600 foot straight you may be raising surface temperatures by a couple of degrees but only scrubbing off 1-2 MPH at the end of the straight.  Those couple of degrees will give you a 1-3 mph higher turn speed in most cases, and 3' or more later braking with front brakes on a full wet track.  It also helps to get the Ackerman closer to what the tire was optimized for. 

Softer Brake Pad compounds give a better feel for threshold braking in the wet as rotors and pad surfaces just won't get the heat they see in the dry.   Front brakes generally require a little more rear bias in the wet to reduce the understeer and allow deeper braking into the turn.  This especially helps if your driving with inexperienced drivers that are likely to spin in a turn as it provides you with a bit more of a cushion to react.

Braking takes a little more alacrity especially with rear only brakes.  Rear only brakes can be used to help pitch the the car to offset understeer on wet turns but becomes very difficult to get a good exit if the attitude of the car is not "on target" at the apex.  Body weight shifting becomes far more important in the wet, especially under braking into a turn.  Most drivers find that tighter seat mounts and looser seat-axle braces help in rotating the car better using body weight shifting. 

Optimal weight balance front-rear and Center of Gravity (CoG) differs from dry to wet.  With the tire carcasses designed for less force to distort (stress) a higher CoG is generally favored.  Because of the understeer problem, particularly at turn-in, you need to get the inside rear wheel to lift as soon as possible (weight jacking).  Moving the seat forward helps relieve the resistance to lifting or weight jacking, and raising the seat back raises the CoG or "theoretical Roll Center".  You shouldn't be afraid to make gross changes in the rain, as raising the seat 1"-1½" is typical.  The down side is it also reduces rear traction on exit slightly, but its generally worthwhile trade-off. 

Another factor to consider in the Weight jacking is the chassis stiffness itself.  Bars -in/Bars-out has a whole new set of rules to play by.  Again, its all down to the grip you have to work with, and the difference in the design of the tires.  Generally, your chassis was designed to be used in the dry.  On the topic of chassis stiffness, each driver varies, as does each tuner and chassis. 

The front and rear bars are more independent of each other.  While they allow the chassis to twist, they do so coupled more to the independent forces imparted to either front or rear.  Removing the front bar may give a little lazier feel to turn-in as during the instant of changing the wheel position the front of the chassis is twisting during the change - but this becomes countered as the car changes direction and cornering forces shift the weight distribution from side-to-side, and the front of the chassis is allowed to accept more weight jacking on the rear. 

Another way to explain this;

At first the front chassis twisting is caused by the castor and is not relayed back to rear as weight jacking.  As the cornering forces are applied, the weight jacking allows the inside rear to lift further as the weight is transferred to the outside.

Chassis Setup

Quick Summary, drowning wet conditions

From here its a matter of what the call is. 

Other electronics such as a Mychron display are not always water proof.  Many people tape a baggie over the display to protect it yet still being readable and leaving access to the buttons.