If your mechanic used the whole D-jet system, it should work OK. Maybe not great, but at least OK.
I suggest starting the FI diagnostic, which means first of all checking out the ignition system. Check the spark, dwell, timing, and make sure that it advances and retards reasonably. While you're doing that, check the plugs to see if the engine is running rich or lean. (I'd personally guess rich.)
Check the fuel pressure. Check the valve adjustment. Then start checking the FI components. Kjell Nelin's D-jet guide on the 914 Fan Page
http://www.914fan.net is an invaluable guide.
Good luck!
--DD