D-Jet Fuel Injection Pulse Phasing Options

[Not_A_Six]

Hi All-

I recently installed a 123Ignition dizzy in my '73 2.0. It apparently can be installed in any of 4 orientations, in 90 degree increments and operate properly when wired to the plugs appropriately.

The d-jet version of the 123 dizzy also has two wires that are used to time the two groups of fuel injection pulses (cyl 1+4 and 3+2). These wires can be connected to the ECU in either polarity, and the "correct" polarity to time/phase the injection pulses seems to depend upon both the distributor orientation, and the wire polarity. Instructions from 123 Ignition show a connector sketch that "suggests" a particular polarity, but I think the correct one also depends on the chosen dizzy orientation, as well as the firing order. (The dizzy can be used in non-914 applications. E.g., with a straight 4.)

I'm an EE. I've been through Paul Anders' ( @pbanders ) very helpful writeups on the ECU and d-jet operation. I've got d-jet schematics. And, I've got a generic (Volvo?) chart purporting to show the injection timing WRT the cylinders (which seems to have a lot of discrepancies with a 914 setup with different firing order.) I've also looked at the injection pulse/spark timing with an oscilloscope (with a high-v probe).

It looks like the orientation and polarity can be adjusted to inject fuel at any of the following times:

1) Inject cylinders 1 and 3 during intake stroke and cylinders 4 and 2 during exhaust stroke.

2) Inject cylinders 1 and 3 during power stroke and cylinders 4 and 2 during compression stroke.

3) Inject cylinders 1 and 3 during exhaust stroke and cylinders 4 and 2 during power stroke.

4) Inject cylinders 1 and 3 during compression stroke and cylinders 4 and 2 during intake stroke.

I think the choice of the 4 options above would affect the dwell time of the fuel charge in the intake runners and might also cause some raw fuel to be sent out the exhaust if a pulse occurs during valve overlap.

Which of the 4 configurations is correct? Is my analysis correct, or did I miss something? Any other advice you can give me?

Thanks for your help.

Cheers. (IMG:style_emoticons/default/beerchug.gif)

[914_teener]

Added plus with the electronic dizzy is the elimination of the trigger points....and yes with regard to the cap and plug position. On the stock dizzy the vaccum can was in an awkward position and because the diaphram had failed and replacement parts my reasoning to use the 123.

Good luck

[Not_A_Six]

Added plus with the electronic dizzy is the elimination of the trigger points....and yes with regard to the cap and plug position. On the stock dizzy the vaccum can was in an awkward position and because the diaphram had failed and replacement parts my reasoning to use the 123.

Good luck

Thanks! Yeah, the 123 dizzy looks pretty sweet. It did force me to choose between vac advance and vac retard, though, as it doesn't have the ability to do both like the OEM one. (I chose vac retard -- profile 'A' -- as otherwise the timing was too advanced at idle to get the idle speed down.)

Cheers. (IMG:style_emoticons/default/beerchug.gif)

[914_teener]

Added plus with the electronic dizzy is the elimination of the trigger points....and yes with regard to the cap and plug position. On the stock dizzy the vaccum can was in an awkward position and because the diaphram had failed and replacement parts my reasoning to use the 123.

Good luck

Thanks! Yeah, the 123 dizzy looks pretty sweet. It did force me to choose between vac advance and vac retard, though, as it doesn't have the ability to do both like the OEM one. (I chose vac retard -- profile 'A' -- as otherwise the timing was too advanced at idle to get the idle speed down.)

Cheers. (IMG:style_emoticons/default/beerchug.gif)

???

Should be using ported vaccum for the advance and no retard or manifold vacuum. I.m wary of your post.

[Not_A_Six]

Thanks! Yeah, the 123 dizzy looks pretty sweet. It did force me to choose between vac advance and vac retard, though, as it doesn't have the ability to do both like the OEM one. (I chose vac retard -- profile 'A' -- as otherwise the timing was too advanced at idle to get the idle speed down.)

Cheers. (IMG:style_emoticons/default/beerchug.gif)

???

Should be using ported vaccum for the advance and no retard or manifold vacuum. I.m wary of your post.

On a '73 2.0 d-jet 914, the throttle body has two vac ports -- one for advance at low load, and one for idle retard. (See ports "A" and "B" on the diagram below.) The OEM dizzy (039 905 205) also has two corresponding nipples.

The 123 dizzy has a single vac port that can be used either for vac advance (e.g., profile "1") *or* vac retard (e.g., profile "A"). You have to choose.

With profile #1, setting the centrifugal advance to give 27 degrees total (w/ vac hose plugged off) at 3500 rpm gives you an total advance of 5 degrees at idle. I couldn't get my idle speed below 1500 rpm with that much advance, even with the idle screw on the throttle body full clockwise. (Granted a vacuum leak could theoretically be the root cause of the high idle, but I've checked/replaced virtually *everything* and am 99% sure that I don't have a vac leak.)

Using profile "A", there is no vacuum advance, but with the throttle plate closed at idle, there is enough retard at idle to get the idle speed to ~1000 rpm. This should (hopefully) mimic the total advance/retard of the OEM dizzy at idle.

If I'm missing something, or have screwed up somehow, please help. (IMG:style_emoticons/default/confused24.gif)

Thanks.