"The gearbox let go. We were sitting seventh and things were looking good," team manager Seth Devereux said from the Florida racetrack.
The Wellington rider, making his United States racing debut, qualified 11th and ran as high as fifth in the prestigious international event.
Although he dropped back to seventh after the first fuel stop, Devereux was confident Richardson was capable of getting back into the top five.
"We were ready to make a big push. It’s just not how we wanted it to end."
Richardson’s impressive run on his borrowed Kawasaki slowed when he lost fourth gear.
"And when it started jumping out of fifth, I knew I had to pull in," he said.
"It’s a shame it ended that way. I was having so much fun. We were going so well, but it is what it is," he said.
