123 Dizzy - with or without vacuum advance?, Sucks... Options

[chmillman]

Last post of the evening, promised...

Last year one of my mechanics suggested putting in a 123 dizzy. I said OK, why not. The model he installed was the dip-switch one with curve #2 set, but without vacuum advance port (plus a coil to match). The vacuum hose was simply plugged. Installing the 123 did not actually fix the hesitation problems we were having, which were most likely elsewhere in the FI system.

My other mechanic here who is a specialist in D-Jetronic stuff looked at that and said "This is not good, it needs at least vacuum advance!" He cleaned, lubed and re-set up the original distributor plus the coil and re-hooked up both the vacuum advance and retard hose connections. He's admittedly more of an 'originalist', says the whole thing is a system designed to work together, so he was not fond of replacing the original mechanical dizzy with an electronic one, especially without a vacuum advance. He pointed out that if one really wants to do that anyway, the 123 units were also available with a vacuum port, and suggested that the other guy simply ordered the wrong dizzy model.

So now I'm here in between these two guys - who are both my friends by now given the stuff we've done together - and I'm no expert in the matter, so I don't know who's 'right'... Possibly both.

[wonkipop]

all VW air cooled engines from the era (60s and 70s) had at least vac advance units to enhance the mechanical advance of the distributor. whether fuel injection or carbs.

you can find a lot of explanations for what the vac advance did.
but the principle one was that at highway cruise the vac advance would provide additional advance mainly to get slightly better fuel economy and lower head temps (run a bit cooler). good for driving for long distances at "high speed" cruise. and you know high speed cruise for a VW is around 60mph. (IMG:style_emoticons/default/biggrin.gif) as soon as you came off throttle or gassed it a bit the vac advance would drop off and you went back to pure mech advance position.
this smoothed things out as you changed throttle positions because cruise is not always steady so to speak. thats really what it did.

vac retard is a whole other thing and mostly its really only on the USA cars of that era.
this is the dual can set up. that retard is what acts at idle with throttle plate closed.
and the retard is there to cut down on NOX emissions at idle. pure emissions device.
but it made engines run hotter (higher head temps). but only at idle did it run hotter.
it also snapped off straight away as soon as you opened throttle to take off. timing went to pure idle setting for timing and car would take off without hesitation. (that by the way is why you tune and set idle on a 1.8 for instance with the vac hoses off and plugged. you time it all and set idle etc in this pure state. then plug the hoses back in and adjust the idle speed if you have to with the hoses on.

thats my understanding of the two functions.

there is a complication in a way wit the dual action can distributors. which is that the total vac advance at cruise is a result of the vac advance - minus the vac retard for resultant vac advance. at cruise with throttle partly open then you are getting vac at both the inlet ports that are hooked up to retard and advance sides of cans. they are both being actioned.

in those 914s with both hoses hooked up whats going on is the emissions are being modified only at idle to reduce NOX. but you are still getting benefits of advance at cruise speed. at that point in time the EPA regs and standards where only reducing NOX at idle.

the 914s that only had the vac retard hose hooked up and the advance hose disconnected where not getting the benefit of the vac advance at cruise.
they were retarded at both idle and cruise. for emissions purposes.

in the case of the 1.8s thats exactly what happened.
in 74 the 49 states had both hoses connected, the californian cars had the advance disconnected. and 12months later in 75 both 49 states and california had advance disconnected and california added EGR.

.........i guess your originality mechanic is argueing they really are in peak form with advance and retard connected. you need the retard connected to get the optimum advance amount in the dual action vac can distributors and you put up with the retarded idle as it does not really do any harm and it reduces emissions without really doing anything bad. and it is all matched to the ECU.

i think the D jets are pretty much the same in the way they use the dual can.
and also at the same time as the L jets did the disconnect advance hose thing. which was a necessary evil for emissions but not ideal for engine temps.


EDIT

i might add i guess it depends how you drive your car.
if you spend a lot of time not cruising but rather hammering around twisty roads, a pure mech advance is probably ok. since your are always hard on the gas or off it.
or a racetrack for instance.

i don't think 911s had vac advance for instance. at least not down here.
but the VWs always did.