[ihc] Dialing it back(Pi?...no, spark!)

["Jim Grammer" <jgrammer@xxxxxxxxxx>]

>Date: Tue, 8 Jun 2004 09:26:51 -0600
>From: "ken.dunnington" <ken.dunnington@domain.elided>
>Subject: Re: [ihc] Dial Back Timing Light

>Al and Nona Evitts wrote:
>AaNE> I just got one of these new fangled lights from a friend.  Can anyone
>AaNE> tell me how to use it correctly?

>AaNE> Thanks   Al

>Hello Al

>Those are the ones to have.<snip an excellent treatise>

I'll add that it's handy to map the vacuum and mechanical advances as
separate curves, as well as the combined curve. The vacuum advance is mapped
at several vacuum increments, as applied by a hand held vacuum pump(Mityvac,
Craftsman, etc..).

Of particular interest is the 'cut-in' point of the vac advance, and how
quickly the advance comes in above that point. There seems to be a fairly
wide range of characteristics among the 'stock' type advance units. The
cut-in point that's often stamped on the pull bar may or may not be accurate
either. On top of that, older trucks like ours are often victims of wrong
parts swapping(looks the same, fits, must be right, right?).

Case in point, '73 T-all 392 - towing in hills, a nice comfortable cruise is
about 50MPH in 2nd gear. 3300-ish RPM, so the mechanical advance is
'all-in'. Manifold vacuum wants to be around 6-8", depending on the grade.
The cut-in point of the vac advance on the truck when I got it was right
about 6". So you're climbing, and can't stay in that 50MPH sweet spot
without the vac advance coming in. Hello, ping :( Sure, you could back off
the static timing or pull back the max. mechanical advance, but the truck's
running great and making *real* good power at that RPM/load :). I found a
vac advance unit in my collection that doesn't cut in until 8" of vacuum.
Problem solved, mostly. I will still need to limit total advance if I want
to tow with 87 octane, or run the leaner metering rods in the AFB even with
89 or 92 octane.

I can't see any reason to bring the vac advance in on an SV below 8-10" of
vacuum, YMMV. All this makes an excellent case for following Greg's lead and
swapping to Delco dizzys, for which you can readily buy mechanical advance
curving kits and adjustable vac advance cans.

Another tip on making timing marks visible. Both the damper mark and the
timing scale cast into the front cover are most easily cleaned from *under*
the truck. Slather a rag with solvent and do the scale by Braille. Go get
one of the new-fangled touch-up paint applicators in your favorite bright
color. They have a ball point applicator as well as a brush. The ball point
is just the ticket for your freshly cleaned damper groove, and if you prime
the tip a bit it'll write upside down long enough to mark the damper from
the bottom. If your arm's skinny enough you can use the brush to highlite
the marks on the timing scale.

Jim