L-Jetronic Fuel Injection and Distributor question, Intake Manifold Vac Ports to/from distributor Options

[Str1974]

I have a 1974 1.8 L with a current Bosch distributor (0 231 181 009). I have two vac ports on my throttle body , one that goes to the distributor (vac retard) and one that comes from the distributor (vac advance) back to the throttle body.

I'm replacing my old distributor for a new 123ignition w/Bluetooth adjustment. The 123 dizzy has an optional port on it for a vac line, so therefore one of my vac lines going from and towards the intake manifold would need to be "capped". I'm hearing that I could either chose to cap either the retard line or the advance line or all together cap both ports on my intake manifold and choose to run my new dizzy without any vac lines.

I've also read that "The 914 distributor has a centrifugal advance built into it. This mechanism causes the distributor timing to change as the engine's RPM increases. The advancement of the timing is necessary because at higher RPMs, the engine needs to ignite the fuel earlier than at idle. In other words, if there were no centrifugal advance, then at higher RPMs, the full force from the combustion would impact the cylinder when it was already through the major part of it's stroke. This would result in lost power output. The centrifugal advance changes the timing so that ignition occurs earlier. Additionally, the fuel injection may advance or retard the timing based upon the vacuum pressure from the engine."

It's this last line from above that has me wondering how vacuum pressure from the engine works if there are no vac lines going or returning from the dizzy to the intake manifold??? Or , am I completely wrong that the vac ports on the intake manifold / distributor has nothing to do with it.

I'm still a novice and would like to understand more about this topic, or at least be less confused....

[ClayPerrine]

The stock distributor on a 74 does not use the vacuum advance side of the distributor. It only used the vacuum retard side, and only at idle. The throttle bodies were also used on the VW sold engines, and often had the vacuum advance port on them. There should only be a hose on the throttle body port that is closest to the engine tin, and that should go to the vacuum retard. The other port on the throttle body should be capped, and the advance port on the distributor should be left open. And the distributor vacuum retard only changes the timing by 2-3 degrees. The Fuel injection has nothing to do with the ignition system on an L-jet car. The only thing the FI gets from the ignition is the tach signal so it knows the RPM. All of the ignition advance is done via the mechanical advance in the distributor.


Somewhere along the way, someone hooked up the advance side of the distributor. Now every vacuum diagram has it.

[75914-L20C]

Somewhere along the way, someone hooked up the advance side of the distributor. Now every vacuum diagram has it.

If the 75 L-Jet distributor is the same as 74, I'm afraid that "someone" may have been me. My '75 originally had a hose connected to the advance port on the distributor. The other end of the hose was open and tucked underneath the throttle body. It sure looked to me like that hose should be connected to the advance vacuum and the previous owner must have changed that. So I put a tee in the advance line and connected it. Later, when I drew up the L-Jet vacuum advance diagram on Pelican, I included it.

[ClayPerrine]

Somewhere along the way, someone hooked up the advance side of the distributor. Now every vacuum diagram has it.

If the 75 L-Jet distributor is the same as 74, I'm afraid that "someone" may have been me. My '75 originally had a hose connected to the advance port on the distributor. The other end of the hose was open and tucked underneath the throttle body. It sure looked to me like that hose should be connected to the advance vacuum and the previous owner must have changed that. So I put a tee in the advance line and connected it. Later, when I drew up the L-Jet vacuum advance diagram on Pelican, I included it.

The factory put a line on the advance port on the distributor and tucked the other end of it under the plenum. That was to keep water out of the vacuum can. It was never meant to be hooked up.