2 stroke help, please! RPM's lower, then bikes quit?!

This has happened with 2 of my vehicles. A 1997 Polaris 400 Sport quad, and a 2004 Honda CR125 bike. They will start fine and I can take them out for a quick rip, but after 2-3 minutes, the bike/quad will slow down, RPMs will lower, then the bike will die and not start again, and even sometimes backfire. It couldn't be a faulty top end, because the 125 is coming off a fresh rebuild. It couldn't be the carb, because on the Polaris, it has been torn apart and cleaned more times than I can count. The plugs are brand new. Thank you!

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