http://www.nintendo.com/consumer/systems/wii/en_na/ts_internet.jsp?menu=connection
One thing you might want to check and see "if the router's security software (firewall) is turned on, it may block communication that is required for online game play."
And/or it could be that certain "ports on the router might need to be opened through the Port Forwarding feature."
But if the information at the above url and copy&pasted below doesn't help still staying connected in Mario Kart Wii, give a call to Nintendo's 1-800-255-3700 (6 am ~ 7 pm Pacific, everyday) Customer Service number, and the service representative will be able to do some over the phone troubleshooting to determine exactly what the problem is, and talk you through any needed steps to correct this and get you racing again against your friends and foes.
Situation:
- Trying to connect the Wii online for online game play results in error codes or disconnects
Information:
Before playing games online, the Wii console must first be connected to a home internet connection. (How To)
If the Wii is successfully connected online, online game play issues are often caused by firewalls within the router or home network.
What to do:
- Try turning off the router's firewall:
- Trouble connecting to a friend?
If the router's security software (firewall) is turned on, it may block communication that is required for online game play. To fix this, try disabling the router's firewall. Most firewall settings are located under the "Advanced" or "Security" tab in the router settings.
If the problem still persists with the firewall off, ports on the router might need to be opened through the Port Forwarding feature. (How To)
If the above steps have not resolved the issue and the problem is happening with specific friends, the problem could potentially be on their network. Your friend will need to perform these same troubleshooting steps with their home network.