Some 2.5 are fitted with HS4 as standard. The low HP / higher torque one.
I have driven one 3 times now, on front of an BW65 as well just to make things worse!
First drive it wouldn't pull the skin off a rice pudding, Topped out at about 50. I did 75 miles like that. 😞
Found rocker gaps were 200 thou, that is what happens when someone nicks the correct pushrods to fit to a customers engine and fits a set of Vitesse ones he has to hand and then just puts the rocker cover back on after tightening adjusters as tight as he can.
Once sorted it was not bad, and would rev quite nicely, but you could not hear yourself for the knocking from the engine,
Anyway, back to the point, it has a 2.5S engine now, on the same HS4, revs cleanly up to 5000, pulls like a train, and you can spin the wheels pulling out of a junction if the owner isn't with you, so 1.5" carbs can work fine on a 2.5.