[SBillinson]

There may be a gauge available to check the pitch and other dimensions, similar to a thread gauge.

You could instead make a template out of aluminum to fit the gear and then compare it to your belt. Take a photo and send it to Jesel if the two don't seat correctly.

Easier still would be to take a photo of the top gear with the belt on.

Most Jesel belt drives don't have an idler. I've never understood why. I imagine the belt would stretch or create slack between the leading and trailing sides, especially with a lot of spring pressure. Has your spring pressure changed significantly?