[AMC-List] Duraspark distributor woes (258 six)

[Tom Jennings <tomj@xxxxxxx>]

For some months I've been having trouble getting the timing
right on my Hornet's 258. It would either retard-backfire or
ping, with little adjustment range between the two.

2.73 axle, 904 auto, weber carb. I've been running cheapsh*t gas.
It's a Duraspark II (square coil, big Ford cap, etc).

I'm getting 19 mpg, down from a high of 20, averaged over full
tankfuls. (I've recorded every single fillup since May 2005).


I decided to do the job right this time. I pulled the distrib,
which was used but cleaned up by me in 2004. I swapped the worn
shaft (and removed crud from housing I think was blocking oil
movement up).

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.

I blocked off vacuum advance for now, for testing. Set static
timing to 10 degrees, and drove to and from work, stopping to
tweak timing. At the end of the day I measure the static setting.

On regular crap gas (87 octane) it just pings under high highway
load (50 - 75 mph wide-open-throttle) with 8 degrees static;
with the 10L setin that makes 28 degrees total advance (no
vacc advance).

That's a lot shy of the recommended 36 - 38 degrees total!



Today, I filled up with "high test" (91 octane). I was able to
advance it probably 3 degrees more, and it doesnt-quite-almost
ping under the same conditions (hard accelleration, WOT, 50 to
75 mph). I'll measure in the morning's light, but I'm guessing
it will be 10 - 12 BTDC static, plus 20 mechanical, or 30 -
32 total.

This seems short too.

It's also quite temperature sensitive. I've got an excellent
cooling system; 3-row rad with two electric fans. At hot-normal
(eg.  after stop-and-go traffic, fans running) it will ping
lightly; after 65mph cruise in the cool, it runs at low-normal,
no pinging. For testing, I did adjust it to the edge. I think
I've got 195 degree thermostat in there. I may lower this to
180 if it lets me run more advance.



Any suggestions on why I'm unable to run much advance?

It's running great, even with no vacuum advance (which I'll do
later). Should I not worry about the numbers and just tune for
good performance?

(Also in the morning I will re-check total advance to make sure
measurement == expected. I've got one of those timing lights
with the knob to null out timing. Not sure how accurate it is!)




Question: would lean carb affect pinging, much? If it's lean
it's only by one or two jet sizes, probably. I did rejetting
a year ago, but I will revisit it after I untangle the timing.


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).


I'm seriously thinking of writing up a how-to-tune-your-duraspark
from all the info I've found, which include

Ralph Winslow's excellent http://www.amxfiles.com/resource/tech/timing.html, referred to by someone on this list (thanks!)

and this excellent

http://www.fordmuscle.com/archives/2000/03/timing/index.shtml




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