Your 250SE has mechaical fuel injection and
the mixture is controled at start up with a time start valve, starting atomizer, [ on the intake] and rich solonoid. The wires feeding this circuit should be connected for proper operation, however I would look first to see if the start atomizer is leaking by. This is common.
This pump also has it's own oil res. and
dip stick on the lower rear end of the pump.
Very hard to see from on top and very few know it is there as the 280's of the following year incorporated oiling passages from the crank-case. Check the dip stick for proper marked level as low or high will
affect the pump metering.
I recommend re-wiring the system. I have the schematics for this series pump and will send them free of charge if you e-mail me an address.