First long trip in the P38 today (to somewhere I can work on it) - it has a fairly bad vibration above about 55 mph, strong enough to rattle everything that’s loose and shake the passenger seat very noticeably. It doesn’t seem to be coming from the front axle (no steering shake or vibration, definitely not death wobble), and feels too strong for an unbalanced wheel or bad tyre; does it under acceleration, cruising and when lifting off, and coasting in neutral doesn’t stop it. It possibly eases slightly at around 80 and gets worse beyond 90. No evidence of a binding brake, and both engine and gearbox are smooth, with no untoward noises.

Aside from an unbalanced/misaligned prop or a bad UJ, any guesses for what to look at? I’d hoped to dismiss the flex plate given it still does it when in neutral.

sounds like uni joints , you will have remove the shaft to check properly , also go for a small drive just to confirm this , put a bolt in the brake housing or it might drift off.

Tyres hit resonance at around 55-60mph so a rear wheel badly out of balance would cause it too.

Thanks - wheel refurb and new tyres next up, so I’ll see what that does before unbolting anything else…

My props started doing this recently. With four axle stands under, it was easy to fell the loose UJ on each. Only a few hours work to remove the props & change the UJ's.