Hi,
I have a 1995 +4 4 seater so know them well.. given it has had new tyres I expect you can rule out that.
A more likely suspect for the hard ride is the front shocks being in need of replacement. This is something that would not be visible in a service as they may not be leaking.
When I was looking for a +4 I looked at a 1991 4 seater in Brighton that hadn’t been used much. On the test drive the ride was exceptionally hard so much so I had a look at the front suspension as I thought it might of been a seized king pin or something. Given he used Brand Hatch Morgan’s I told him to take it there and ask them to look at it as something was amiss. A new pair of front shocks sorted it instantly, so much so he decided to keep the car!
Would recommend Avos on the front like the others. I bought my +4 from AW as a trade sale, the first thing I did was new front shocks as it was still on the factory original ones from 1995 and a new set of tyres.
Good luck.
Will
