Fuel settings? Perhaps you are only running on one fuel tank, so the balance is out? Large airliners rarely even notice this, as they outweigh the gas by enough. But a small single or twin prop is going to notice big time, especially with full realism. Maybe that? Or perhaps the fuel loads got messed with somehow.
From my stock Mooney:
LeftMain = -4.1, -7.9, -0.8, 47.5, 3.0
RightMain = -4.1, 7.9, -0.8, 47.5, 3.0
fuel_type = 1
number_of_tank_selectors = 1
electric_pump=1
And you might try redownloading the Piper to see if there is something awry in there.
But then again, it could be a gauge problem . . . that would be a real bugger. If its that, and the Mooney is affected, that means a stock gauge set got replaced. You'll have to get into the disks to find it again.