i have a 91 xr4 black max and think the lower unit has gone out it will put about 5mph in forward drive at low rpms and just rev high and go no where when i throttle up
That could be a spun hub in the prop. Change it over to a known good prop or mark the brass hub and propeller with matching marks and reinstall and test prop. If marks no longer match, prop needs to be repaired.
