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

[JeffBowlsby]

I had answered this previously in this same thread:

"The two port throttle body is a 73 2.0L version, the 74 only has a single port - nothing to cap off on the 74 version"

The ignition advance was not connected to the throttle body on the 74-76 2.0L cars. From the factory, a short length of hose ran from the advance port and was routed under the air plenum not connectoed to anything, which is what my diagram depicts

[wonkipop]

I had answered this previously in this same thread:

"The two port throttle body is a 73 2.0L version, the 74 only has a single port - nothing to cap off on the 74 version"

The ignition advance was not connected to the throttle body on the 74-76 2.0L cars. From the factory, a short length of hose ran from the advance port and was routed under the air plenum not connectoed to anything, which is what my diagram depicts

good stuff mr. b

interesting the D jets were just like the L jets then.
makes sense.
(IMG:style_emoticons/default/beerchug.gif)

in that state with the retard hose only the distributor will retard the ignition whenever there is engine vacuum.
thats either with the throttle closed at idle or when the engine is part open throttle at cruise or deaccelerating with the throttle snapped shut.

when the advance hose is hooked up it can only affect the distributor when the throttle is open. you get engine vacuum at cruise that will operate that side of the can.
the total advance is a nett figure. vac can advance - vac can retard = total vac advance added to mechanical (centrifigal) advance.
thats the way it works. thats why the vac advance side of the can is larger than the retard side.

hence my view that the versions which utilise both sides of the can -
that would be a 73 2.0 (thanks mr. b) and a 74 49 states L jet - are probably better from a driveability fuel economy point of view. you would have to drive the two versions side by side to notice the difference and i'm guessing its really only detectable out on the highway at cruise. you would get better fuel mileage and probably run a little cooler.

[JeffBowlsby]

In the wild and crazy portion of my mis-spent youth I once tried the 1973 2 port TB vs the stock 1-port on my 74 2.0L 'performance upgrade'

The performance difference was awfully subtle. Maybe in a lab hooked up to wires and test equipment there is a difference but not the butt dyno. Underwhelmed if not unnoticeable.