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Problem identified and solved! The arm on the gas pedal bracket has apparently bent over time, so its throw was shortened and was not creating enough tension on the transmission belt. In the image, new assembly is on the left, old on the right. The highlighted green line is where the arm...

You can also see on the top belt that goes to rear axle, the wear marks are not moving full out to the edge of the pulley. Versus on my brothers, it's shiny to the edge:

Here's a video my brother took of his pulley, the upper belt is for sure getting moved out on that variable speed pulley much farther.

Thanks for the info! As mentioned in original post, I did replace belts with OEM and the tranny tension spring. I've since even sanded down pulley faces to reduce any glazing.

Here's a video of the variable speed pulley movement, seems clean/complete.

Model: 13WX78KS, 2/2011 Only getting about 2.5mph out of the mower forwards/reverse. I've been lurking as this is a common problem but I think I've hit all the major points. New MTD upper/lower belts, and the routing is correct. The variable speed pulley seems to be working, but I'm starting...