Starter weak when ATV is cold. Battery tested OK @ 12.1v. When ATV is run a bit, starter works fine. Cleaned battery connections HELP!!

 

A battery with higher cca would be my choice here.

 

Try to remove the battery and put a good top charge on it, you may already know this, but just because you show 12.1 volts doesn't mean that you have the correct amperage under load. The reason that it starts good after you ride it for a while is because the atv top charges the battery and increases the amps. If the charge doesn't work, then I would take v2riders advice and go after a new battery. I hope this helps.

 

12.1v under a load? Or 12.1V static? I've seen many batteries that will say 12V or higher with a volt tester but will drop to almost nothing as soon as you put aload on it.

Your battery is probably shot. Replace it. It's a cheap easy fix.