[T3] 72 FI simplified

[R. David]

Whenever there is talk about setting the ignition timing on a ’72 FI system,
it is considered a pain in the butt, and it is, and I want to try and
explain why. Our Type 3 is a ‘72, and every FI component on it is the
correct ‘72 part, so I have some experience with this system. That and I
asked a lot of questions to this website! I hope I explain this correctly,
but the FI experts here can correct me if I don't ;-)

As with any FI ignition timing procedure, you need to have the common tune
up items set correctly such as point gap, spark plug gap, fuel line
pressure, and so on. Bentley correctly states that the throttle plate must
be fully closed. This is because if the throttle plate is not fully closed;
this system won’t work as designed.....period. This also means your throttle
position switch must be adjusted correctly so it is not a factor in holding
the throttle plate open. Also when you set the timing on our cars, make sure
the motor is at operating temperature so the auxiliary air regulator is as
closed off as it is going to get.

What makes the 72 system different is it has a double vacuum advance
distributor. One port of the vacuum advance works like any other vacuum
advance distributor where it advances the timing when rpm increases, but the
second vacuum port works when the engine is at idle (hence the throttle
plate needs to be closed). Under regular driving conditions; At idle, this
vacuum port moves the points mounting plate ever so little, but this
movement, combined with the idle speed screw, is what sets the running idle
speed. If you remove and plug this second vacuum port on a properly set 72
system; your idle will increase several hundred rpm! Getting your idle back
down to 850 rpm (proper speed for setting timing) WITH the vacuum lines
disconnected and plugged (on the IAD side) is the important part when timing
the 72 system.

Again: after you remove and plug both vacuum lines to the distributor; your
idle speed will increase. Now lower the idle with the idle speed screw back
down to 850 rpm for proper timing setting at 5 degrees BTDC. After the
timing setting is complete - re-install both your vacuum lines and readjust
your idle speed using the idle speed screw. That’s it! (sarcastic snicker)

I’m sure I probably left something out, so please feel free to correct me if
I messed something up :-)

Dave Pallo
'72 Square ~ "Elwood"
Fairport, NY