Watch as I diagnose the smart drive self propel system on this Honda 21'' self propel mower. Right now, the cable looks like it has already been adjusted longer and I adjust it further with no improvement. The belt tension looks good and everything else looks to be in order. That just leaves the OEM transmission as the faulty part.
Watch me remove the variable speed transmission
This job is best left for a better skilled DIY'er or I'd recommend have a pro do it. Some semi complex disassembly and reassembly required.

Amazon links to products I use and recommend
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Digital Battery tester
https://www.amazon.com/SchumacherBT1...

Battery terminal cleaners and fiber washers
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High energy ignition tester
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Vampliers
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Carb cleaner
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Carb dip
https://www.amazon.com/BerrymanProdu...
Tireject tire sealant
https://www.amazon.com/LawnMowerTir...
Evaporust rust remover
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lock n lube grease gun nozzle
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Fluid film rust preventative lanolin based and awesome!!
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NOCO jump starter
https://www.amazon.com/NOCOGB70Ultr...