Must I really change the backplate and the intake manifold?

If you use the backplate of the FS91 with Pump the new carburetor would only be a little higher because the pump version of this engine needs a little more space in the backplate area for mounting the pump unit.

Without changing the backplate you can use the original intake pipe which is a little shorter then the intake manifold of the non-pump Version.

But the manual shows also different main needles.

Best regards
Robert