Re: [AMC-List] Duraspark Distributor woes

[Tom Jennings <tomj@xxxxxxx>]

On Wed, 25 Oct 2006, John Elle wrote:

> Snip
> For some months I've been having trouble getting the timing
> right on my Hornet's 258. It would either retard-backfire
> Snip
> This is the problem I have had with YF carburetors over the years, but
> only on standard transmission applications and is directly attributable
> to the vacuum operated accelerator pump. Apparently the flexible
> coupling of the torque converter alleviates this symptom when used with
> an automatic. 

Well there's lean backfire, and timing backfire. I can now hear
the difference, I think; the spark/timing backfire is louder,
and continues as long as the conditions remain. (Fuel:air ignited
by errant spark.) Lean backfire is sharper, "drier" sounding.

I most definitely had timing backfire, lots, and it was
adjustable and repeatable by changing the timing.


> SNIP
>  Or ping,
> SNIP
> In my opinion this is 98% related to compression ratio and gasoline
> octane rating. 

It may simply be a lot of this, and that is that. But I haven't
given up yet!

> SNIP
> It had a mechanical advance unit that did only 13 degrees
> advance!  (I didn't know about the markings on the thing back
> then.) I had another distrib that contained a 10L/15L unit, so
> I installed that, set to 10L (for 20 degrees total mechanical
> advance).  I'm running the two light springs from a recurve kit;
> all advance is in by 2500.
> SNIP
> I have not got a clue what you are talking about here, it must be unique
> to the Distributor u are using and although all timing in by 2500 sounds
> a bit early to me, it probably can be gotten away with. The operative
> word is probably.

First, I didn't know this until I stumbled on the FordMuscle web
page, but Duraspark mechanical advance units are marked with the
amount of advance they generate, and further, that there are two
slots, and you can pick either. The number stamped on it ("10L",
"15L", "6.5L", etc) is HALF the number of degrees of ignition
advance (eg. it's the # degrees of DISTRIBUTOR rotation).

I have photos.

Second, here's the state of affairs as far as ignition timing
in our old distributor (open-loop) cars go: factory settings
(low initial setting, very slow and late mechanical advance)
was designed to work on 100% of the product population; no
harmful pinging, smooth performance, no-brainer maintenance,
and never claimed to be optimum. Those settings minimize customer
and service complaints.

For performance (maybe not for emissions? I don't know) and
mileage for our vintage american engines they run better with
maximum advance all in by 2500 - 3000 rpm, with the initial
setting determined by experimentation (precisely what factories
don't expect customers and servicers to do) you can get superior
performance, on a per-engine basis. Since we're car nuts,
we don't mind the extra fiddle-time to work out the details.

> It is at this time I would pull a few service manuals to refresh my mind
> with facts but at 2 am and can't sleep I'll do that later. 

I have gone over six cyl distributors, over the 60's and 70's,
and the advance curves vary all over the place! I made a biug
chart a few years back but I seem to have lost it. I may look
for it again.

> SNIP
> I blocked off vacuum advance for now, for testing.
> Snip
> Tom, I think we both know what vacuum advance is and does but when
> working on a problem I tend to re-define vac. Advance as vacuum retard.
> In essence what ever the vacuum is at any given time more advance is
> dialed in, but at zero vacuum advance caused by vacuum is also zero.

Yeah, now that I have mechanical advance pretty close (for
current circumstances) I was surprised to see that connectin the
vacuum advance did not affect highway pinging. It's definitely
working, and parked, revved to 200 or so (by ear), I get 40+
degrees advance. But on the road, under load, it will never do
2000 rpm with 20 in hg vacuum, so advance will drop under load,
as desired.

I don't think I'm gonna need an adjustable vacuum advance
unit. 

> SNIP
>  .......
> That's a lot shy of the recommended 36 - 38 degrees total!
> SNIP
> Once again, this is an area where I would check a couple of TSM's for
> facts, but loosely speaking, one half of the advance would be
> mechanical, the other half vacuum. 
> Ergo, with 38 degree total, 19 degrees would be the mechanical advance
> with a 10 degree setting and at 75 mph advance would be about 29 degrees
> and to my way of thinking at that rpm you could be at a really iffy
> area. 

That's not how I read the current recommendations; "36 - 38
total" means static setting + mechanical advance maximum. Vacuum
advance is highly variable, and "on demand", and partly
closed-loop (load, and your foot :-)

With absolute maximum engine vacuum (decelleration, close to 30
in hg) at high rpm, you get up to 50 degrees advance -- but never
when the engine is under load, since then, vacuum will be low,
you'll never get max. vaccum advance at load.


> SNIP
> Primary I htink is OK, by reading plugs, but I may have my
> secondary too lean (it's a progressive, mechanical linkage). If I
> rich it up, will it reduce pinging? The secondary is only open
> during hard acell; it cruises at 70mph on only the primary
> (you can easily feel it in the pedal).
> SNIP
> I Don't know on your weber but on a 4bbl unless you have your foot in
> it, the secondary is not be used under the conditions you are
> describing. So it would make no difference what it is jetted at. This is
> because secondary operation is usually some form of a progressive
> situation and light throttle cruising should not have it energized. 

The secondary throttle opens mechanically, when the primary
is about 2/3 open. It then opens fairly rapidly. You can feel
it in the pedal.

Cruising flat and level up to 70 mph or more, the secondary
is CLOSED. But under wide-open-throttle the secondary really
matters, and might be lean. But it was pinging on the primary
venturi only, and I know that is not lean. Plugs are nice
cream-in-coffee, insulators perfect.

> is why I prefer a small 4bbl for an I6 rather than a big 2bbl. That is
> also why you can get away with a 500 cfm Carter but on a Holley you have
> to be a whole lot closer and on the money 'cause secondary control
> sucks. A 400 Carter would be better. The term get away with becomes the
> operative term.

Yeah, the Carter/Edelbrocks are nice, but I think the Webers
are better for non-race sixes.

I'm running a 32/36 DGEV; that's 32mm primary, 38 mm secondary.
It's 300cfm total flow, so primary is about 130 cfm, secondary
is 170cfm. Low-speed response is better than the little Carter
YF, yet wide-open-throttle has enough velocity to pickup fuel
(and not bog like a 500 cfm carb). There's a 34/34 and a
38/38 too.


They are/were really popular on Jeeps. There's a $30 dapter to
bolt to the BBD pattern, and a throttle linkage kit that snaps
right onto the stock AMC linkage. Air cleaner is a funny Weber
thing. Electric choke, manual available.

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