D-Jet vacuum hose question, Which hose, or Both? Options

[Olympic 914]

Just finished up installing all the FI wiring harness and the vacuum hoses and now the engine compartment looks like a bowl of Spaghetti.

This engine started out as a 73 1.7 with the stock FI and I converted everything to a 2.0 system. I did use some of the original 1.7 parts as they were the same for both engines. But all the 2.0 FI parts ( throttle body, plenum, air filter housing ,etc.) where cobbled together from fleabay purchases and I am not sure what years those parts were from. Also used the original 73 1.7 FI wiring harness and ECU and had to make a couple extensions to get everything to hook up. Not much problem there, (except for trying to connect those ground wires under the plenum. )

Now to the question, I was using Bowlsby's vacuum diagram that is for a late 74 2.0 system and while hooking up the vacuum lines to the dizzy it seem that one line is no longer used.



in the diagram above I have the green vacuum hose connected from the throttle body to the dizzy as shown by the Orange circles. I also have another vacuum port on my dizzy that I have connected to the second vacuum port on my throttle body shown by Blue circles.

Should I have both of these hooked up? or do I disconnect the Black vacuum line as shown in Bowlsby's diagram and just plug the second port on the throttle body.
This is what I have hooked up now.



Is there a reason that the later versions did not have this second port hooked up?
What benefits are there to using the second line to the dizzy vacuum canister?

in picture below the green line from the throttle body is routed differently

[wonkipop]

@Stratfink i suspect this is a bit similar to the L jets.
D Jet experts like @JeffBowlsby will chime in - they know for sure.

74 49 state L Jet 1.8s had the dual port throttle body hooked up to both the vac advance and retard sides of dist. vac retard went to idle port on TB. vac advance went to port in front of throttle plate.

the vac retard dealt with emissions at idle basically. lowered NOX. but ran hotter at idle.
the advance gave fuel economy and cooler running at cruise. did nothing for emissions.

the 74 cali L Jet had single port throttle body (or one of the ports capped).
only the vac retard was hooked up to the throttle body. (to idle port).
this meant lowered NOX and idle and additional slightly retarded ignition timing at cruise.
lower NOX hotter running at cruise.

california had higher emission standards in 74 than 49 states.

in 75, the 49 state L Jet ran the 74 californnia set up.
and the california 1.8 hooked up the EGR to the port on throttle body in front of throttle plate. the calif set up gave lower NOX at idle via the ignition timing, the EGR restored fuel economy and cooler running at cruise despite the slightly retarded timing, which served to lower NOX.

i imagine its similar for the D Jets.
though i am not sure it was split between 49 states and california like it was for L jets.

the double vac can distributor and ignition timing played a large part in emissions control during those years.

there is a bit more detail in the 74 L Jet thread in the originality section of this website.
it took us a bit to work it all out for the L jets.