Why You Need to Waterproof the Electrical System on a Dirt Bike
For One, Just washing a dirt bike often allows water to get in places you don't want, such as the electrical connections that make it run, and if you haven't learned the hard way yet, having water in your electrical connections can cause multiple problems such as misfiring and / or a high resistance condition, which can damage electronic components, suddenly leaving you with no spark at the worst possible time, like when you're leading a moto, or when you're at the furthest point from the truck in the woods.
There is an article elsewhere you may want to take a look at on washing a dirt bike as this has a lot of good tips for getting a dirt bike clean and displacing the water afterwards, but every now and then, you still need to focus on waterproofing your electrical system, so as to have a dirt bike that will keep firing after the next deep water crossing, such as what can be commonly found off-road, or what's forming between the giant double or triple that you're going to have to roll on a motocross track because it rained all night the night before your race, and now there's no traction so as to be able to get enough drive to clear it.
Waterproofing the electrical system is generally accomplished by sealing electrical connections with a dielectric grease (which can be found at most auto parts stores), and taping harness looming with electrical tape, so as to prevent water and dirt entry between leads, as well as applying dielectric grease to the high voltage contact of the ignition system, all of which I will be discussing below.
How To Waterproof the Electrical System on a Dirt Bike
Remember This... Although most electrical connectors are a weatherproof style connector that is designed to keep any moisture out, waterproofing the electrical system is still a good idea, and will insure that all the connections retain excellent connectivity, even in the wettest and sloppiest conditions. Additionally... If you notice any terminal pins that are broken, or are badly corroded, you will need to replace the affected harness or component(s) before continuing.
One of the best and most basic ways of waterproofing your electrical system is to locate each and every push style electrical connector (which there are quite a few of on the newer EFI dirt bikes), disconnect each electrical connector one at a time, then use compressed air to "blow-dry" both the male and female terminals (even if they don't appear wet) then apply a coating of dielectric grease to each pin's female cavity in the electrical connector, then reconnect and secure that connector before moving on to the next.
Note: The aforementioned can be done effectively by just squeezing out enough dielectric compound to cover all the pins, and spreading the compound about, working the dielectric grease into each cavity with your finger, and is an excellent start to waterproofing your electrical system.
Note: You may need to remove some components such as the fuel tank, radiator shrouds or seat to allow access to the entire electrical harness and electrical system such as EFI / CDI / ICU connectors or the "TPS" (throttle position sensor), "ECT" (engine coolant temperature), "IAT" (intake air temperature) and others as some connectors may be hard to access otherwise, along with many other components not listed such as the actual fuel injector's electrical connection and fuel pump connector as an example.
Additionally, removal of these components will likely be necessary in order to access and remove the screw(s) securing the ignition coil on some "coil over plug" style ignition systems, as this will be necessary so as to be able to apply dielectric grease to the inner part of the spark plug boot and should be done.
Once the electrical connections mentioned above have been coated with dielectric grease, be sure to look the dirt bike over for additional electrical connectors, such as tail light or headlight connectors on "X" model bikes designed for off-road use, starter and related electrical connections, engine stop connectors, neutral switches or any other part of the wiring harness not mentioned, applying dielectric grease to all push style or 2-part electrical connectors.
Regarding the ignition system:The ignition coil on dirt bikes with a "coil over plug" should be inspected for signs of firing through the sides directly to the cylinder head, as this will be indicated as a centralized "whiteish" or black appearance on the outside of the insulator. If there is any discoloration noticed, it is advisable that you replace the ignition coil at this time.
Additionally, On motorcycles with the spark plug deep in a well that has a spark plug boot which also fits snugly, and is designed to seal at the top of the cylinder head, or valve cover, so as to prevent any water or other dirt and debris from finding it's way into the spark plug well area, these boots should also be checked for any white or black appearance as previously mentioned and replaced the ignition coil if any signs of firing through the sides are detected.
Lastly, If oil is present on either the spark plug boot or ignition coil insulator on the type of ignition systems just mentioned, it is important that the oil leak is addressed and the contaminated plug boot or ignition coil is replaced to prevent future misfiring.
If You're On A 2 Stroke and You're Still Running a Stock Plastic Ignition Cover, Replace it with a Much More Durable Ignition Cover to Seal the Water Out.
How To Tape the Harness to Keep Water and Grit Out
Taping of the harness with electrical tape is also a good idea but needs to be done carefully so as to pull the tape tight with each wrap, otherwise pockets may be inadvertently created where water can accumulate and find it's way down inside the sheathing of an electrical harness possibly causing future problems.
Areas that are good to focus on taping, so as to further waterproof your electrical system are where the OEM harness sheathing ends, and leads branch out to other electrical components, as these areas sometimes have a rather large opening which will allow water, potentially carrying sand or dirt particles to find their way down the length of a harness, possibly damaging insulation and causing future problems which may prove difficult to track down.
If You're on a 2 Stroke...
On most 2 stroke dirt bikes there is a plastic cover that covers the stator / generator / magneto / pickup and flywheel assembly and this cover, (usually plastic) is common for distorting due to the heat cycles it endures, which then allows water entry when washing and / or crossing water during a ride or race, whether this be on a track, or a trail, it really doesn't matter at this point, water is water and it will get anywhere that is not sealed tightly.
The solution for this is to install an aftermarket ignition cover such as a Boyesen Ignition Cover, as aftermarket ignition covers are generally much more durable than the stock plastic units they replace and will seal much better, effectively keeping all the electrical parts inside dry.