The '89 XJS vin 163*** just ran great, then I made a left turn after a stop when the car would not go over 25 mph. If given more throttle , it surged heavily and slowed a bit. Limped it home and have

checked both coils, dist cap and rotor . New plugs and wires. Noticed strong vacuum felt at left air intake, but little at tight ! Cats were replaced with empty pipes , no difference.

Any ideas ?

 

With a 163 VIN it is a Marelli.

I would swap the plugs between the ignition modules on the top of the radiator and swap the inputs to the coils. This will run B bank on A banks ignition module and vice versa. BTW RH bank is A.