[T3] odd issue RPM hangs.

William Jahn willjahn975 at gmail.com
Mon Oct 14 15:06:01 PDT 2019


Jim I see why the 5* BTDC needs to be added because you are already 5*
ahead of TDC.

On Mon, Oct 14, 2019 at 1:40 PM Jim Adney <jadney at vwtype3.org> wrote:

> On 14 Oct 2019 at 11:57, William Jahn wrote:
>
> > Jim , on the mechanical advance how did you come up with 21 degrees
> total.
> > I only have the Bentley yet it shows 6 degrees yet this is dist speed on
> a
> > machine so you double dist speed . I see the 72 dist chart dist installed
> > and it's near 21 degrees and has no graph . This means to me you can't
> > double the RPM and come up with 12 degrees. Or do you have another source
> > you get the full advance or is this what you have checked with a light
> and
> > recorded? I assume total being control in degrees dist shaft begins when
> > the advance first kinks in at RPM  not from the set 5 * BTDC or are you
> > just talking about 21 degrees is as far as the mechanical advance should
> > reach?:
>
> Looking at the Bentley recently, I just noticed for the first time that
> all the
> graphs are in distributor degrees while the charts for '72s are in
> crankshaft
> degrees. Bentley doesn't specify, but that's the only way these make any
> sense.
>
> The 6 degrees you mention for '73 is the vac adv, averaging closer to 5
> deg.
> The middle of the mech adv is ~10.5, which I doubled to get 21 crankshaft
> degrees of mech advance.
>
> To get the actual firing timing, you have to add the 5 crankshaft degrees
> BTDC at idle, plus any vac advance that comes into play. What this works
> out to is a max 26 deg of crankshaft advance under med or full load or 31
> deg under light load (cruising) when the vac advance kicks in.
>
> The FI vac advance only comes into play under med or high rpm and light
> load. We don't get any vac advance at idle or when accelerating or during
> hill climbing. It's only while the engine is "loafing" at highway speeds.
> That
> improves fuel economy on trips.
>
> --
> *******************************
> Jim Adney, jadney at vwtype3.org
> Madison, Wisconsin, USA
> *******************************
>
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