[morris]

Looks like bad news for the moment.

I've just run the car with the back end on stands and with the rear wheels on or off, you get the bad vibration at around 40mph.

I left it running on idle in 5th and from what I can see of the prop past the exhaust, it didn't look that smooth. Maybe some run out around the slip joint area.

the good news is that the drive shafts didn't visually seem to be vibrating so the problem must be between the engine and the diff. There's a little vibration when rev-ing the engine through 1000-2000rpm but nothing to get worried about. It certainly doesn't cause the issue when reving through the range in neutral which I guess rules the flywheel and clutch out too. So I guess it can only be the gearbox, prop or diff. The former will be easy to rule out if I disconnect the front end of the prop though I'm pretty sure I tried that when I first got the engine running but before installing the prop and I would remember a vibration like this. I don't remember any vibration from the diff on my drive home after buying the donor either.

Tomorrow I'll get it a bit higher in the air so I can look underneath properly, maybe drop the exhaust and try and film it at the resonant point to see what's going on. I'm hoping it is something silly (but not funny) like a bolt working loose though I can't quite see how that would have happened so soon.

If the prop does look dodgy I guess it's easy enough to send it off and get it looked at.