EFI and mileage

[Jason Greene]

Hello to everyone.
I'm new to the list and I have a question.

I put EFI on my 72 Datsun 510. The car never actually came with EFI. I used an
ECU from an 81 200sx and the manifold from a Japanese model car.

Well my question is about my mileage.  Why did my mileage go down by almost 1/2
(30mpg down to 17mpg) when I went from the carburettor to  the EFI?  I've
played with the Air/fuel mixture and it never improves.  My engine size is a
2.2L and the injectors came from the same size engine that had a cross flow
head (mine doesn't have crossflow). Would any of this make a significant
difference?

I know of 2 other guys that have done the exact EFI conversion and get great
mileage. I've tried replacing the ECU but it didn't make a difference. Does
anyone know of a sloution or any ideas thatI can try?  Please help......

Thanx in advance,
Jason G.