[T3] FI Dizzy for Carbs

[Jim Adney]

On 10 Jun 2014 at 20:12, Max Welton wrote:

> The parts aren't new and the combination isn't stock. So we're going
> to take some actual measurements and go from there. 

If you don't have the Bosch charts for the vacuum advance, that's 
something you need to do, but getting hold of the charts would be a 
LOT easier and save you a lot of time. Most of the charts are in the 
various Bentley manuals, and I've never found the vacuum advances to 
change significantly with age.  

Mapping the carb vacuum port output is even harder, but I don't know 
any way around doing this yourself, unless you can just use the 
advance system that VW paired with it. It's important to understand 
that not all intake systems give increased vacuum with increased rpm. 
Many do not; for systems that came with both mech and vac advance, 
the vacuum port often gives no vacuum at idle, max vacuum at low 
throttle (cruising), and min vacuum at full throttle.

It all depends on where the vacuum port is drilled with respect to 
the throttle butterfly, and this varies immensely.

> We're going to find out how many inches of vacuum Adams carbs generate
> on his engine at this altitude under partial throttle openings. Then
> we will work out how many degrees of advance this particular
> distributor turns that signal into (degrees per inch of vacuum). We
> will also measure the centrifugal advance range of this particular
> distributor. 

You should be able to get the mech adv chart from the appropriate 
Bentley manual. Those don't change with age either, unless it's been 
tampered with. For the vac adv, you'll also need to take it out on 
the road to get reliable partial and full load readings. You can't 
just look at the driveway readings and extrapolate.

> It's kind of like when you rebuild a speedometer. When you have the
> unit right there on the bench, you can test it to see if it is
> accurate. You wouldn't just assume it is accurate because the needle
> moves, right? 

Unfortunately, that's all I'm able to do. I can fix the typical 
odometer problem, but the equipment to recalibrate the speedo would 
cost thousands and take up space that I don't have (and my wife 
wouldn't tolerate.) Fortunately, That's never turned out to be a 
problem. I've done lots of before and after tests, and it's clear 
that neither disassembly nor age changes the speedo calibration. The 
odo calibration is done by gearing, so that was set in stone when VDO 
built the speedo. 

I have, on a couple of occasions, used my drill press to provide a 
fixed input speed for a speedometer that needed unusual repairs to 
the needle return spring. That enabled me to compare with an 
untampered speedo and get the speed calibration correct. That was 
time consuming and fiddly, so I hope to not have to do that very 
often.

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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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