20hp ohv inteck will not start

i was cutting the grass and the mower started bogging down and died. it has gas and the battery is good, i had the starter tested they said it was good but is was at a auto parts store so i can't be 100% postive on it but the gear did ingage when they tested it. i even tried to jump start it with a car with no joy. it tries to engage the starter when you trun it over but it only turns the flywheel about two teeth.
this is on a LT2000 craftsman rider 42" cut deck