LowTime, the fCU does not introduce the fuel on initial start but supplies the start electro valve on top of the engine which supplies the fuel injectors. If you start your arriel and look carefully you should see 2 phases to the start. The first is a t4 rise to about 300-350 degrees where she stops for a split second-this is the igniters and fuel injectors working like a lighter in getting heat and some flame going for the next stage. She will then climb for the second phase which is when the FCU supplies the injection wheel and the self sustainable flame is established and the acceleration on the 1S1 modulated if required and/or on a very good FCU controlled by the acceleration control unit and overspeed and drain valve within the FCU utilising P2 bellows and fuel differential pressures which will be related to N1 due to the fuel pressure variances in the pump speed. That is my best attempt to a very short explanation of the system so please maybe ask your engineer to show you his training manual with the fancy drawings and you will understand easily.
Are you sure it is too much fuel and maybe not to little? The 76 does not have airframe booster pumps like most other models(used to prime the engine before start for the Arriel 2 and Arriel 1 for priming and assiting the single engine driven pump) using the Arriel. They have the venturi on the supply line just below the engine above the deck assisting the engine to suck the fuel but to be effective it is critical that the check valves mentioned works as it needs fuel in the line to be effective and then obviously N1 so the pump can initiate the venturi effect. This is why you have the function of priming 1 engine that would not start if the other is online by using the online engines fuel p to prime the problem engine. Ask your engineer as he can tell you that if you disturbed a engine supply fuel line you have a manual bleeding process before the next start to ensure the lines are primed.
The C+ uses the 2S1 engine. The big difference is FCU hydromechanical vs HMU DECU controlled. However, the 2S1 will have the same symptoms if the airframe fuel system supplying it is not primed with fuel for whatever reason. If both engines does it regardless of which one you start first, maybe have a look at the priming line between the 2 engines as it might be allowing air into the system when you start the first engine.
Hope this helps but I am sure Nick can give a much better explanation and look forward to the solution to your problem-so we all shall learn.