Very little I imagine.

That D-jet system needs a bunch of extra timing and a fuel/timing combination that gets 17in of vacuum at 800 rpms and you will get decent mileage.

I'd start with about an extra 7 degrees of timing and if an exhaust gas analyser isn't available go lean untill it runs bad and then go rich until you first reach highest manifold vacuum, hopefully 17in..

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician