Ignition timing set-up

[Matt Sale]

> 
> Help ... I've done a lot of work rebuilding a Ford 351W for
> a Bronco and am experiencing similar problems to what I had
> when I first pitched the old variable venturi carb and EGR
> for efi and a regular distributor and DuraSpark ignition
> off a different vehicle.
> 
> Here's the problem -- timing!  (at least that's what I *think*
> is the problem)
> 
> If I set the timing to give good low end response (requires
> 20 to 25 degrees advanced at 600 rpm, no vacuum), I get
> clatter under load at high rpm's.
> 
I've been battling much the same problem with my 351W 
DuraSpark/MSD-6A combo.  I run a fairly warm cam (Comp-cams
dual-pattern hydraulic), & recently switched from a Holley
650 to an Edelbrock 750.  I've tried both ported & direct
vacuum advance.  I've turned the vacuum advance adjustment
screw to its limit for least advance, and run direct
vacuum presently, with about 10 degrees idle advance (mech).  I 
ended up setting the carb fairly rich to reduce pre-ignition(?)
at part throttle accel (like trying to maintain speed on
an uphill grade in 5th).  Then my O2 sensor says I'm a bit
rich on level ground.  Playing with the timing light in 
the garage, I see way beyond 30 degrees advance at 3000 RPM
with the vacuum hooked up, causing pre-ignition.

Like you, best idle is obtained with about 20 degrees 
advance (vacuum + mechanical).  I wouldn't recommend
setting idle mechanical advance beyond 12 degrees, or
your total advance at 3000 will be over 30 (no vacuum),
and thats not good.  Although idle and startup suffer,
I've had my best high-rpm operation with the idle
advance (mechanical only) set around 6 degrees.  Again,
this keeps the ignition from being over advanced at
high RPM.  I have a terrible time with idle at 6
degrees though.

Realistically, most of my problems are due to attempting
to operate the engine outside its optimum point.  I have
a cam, carb, intake (Torker-II), and heads (Dart-II) that 
are intended for 3000-6000 RPM, yet I want to cruise at
2000 RPM in 5th.  If the Vettes and Camarobirds can run
so well and cruise at 1500, why can't I, right?  The
reason I can't is that I don't have EFI and EST, and
to obtain my HP, I've compromised the low end.  With
megabucks of engineers, computers, labs, etc GM can
analyze everything to the last nano-whatever, and balance
the whole system.  You and I are just hacking around
the edges, experimenting by trial-and-error, with a
very limited number of trials.  

Supposedly you can take the distributor apart and bend
some tabs to limit total mechanical advance.  I've
tried, and found the tabs very hard to bend.

I haven't tried hotter plugs, but maybe I should.

So whats this have to do with EFI?  Only an understanding
of what works and what doesn't, and why I need an ECM!

Oh, and another reason not to set your idle advance so
high is wear and tear on your starter.  I've burned up
a few starters trying to start a warm engine with too
much advance.


-- 
Matthew D. Sale,  IC Development Engineer, Delco Electronics Corp.
msale at iquest.net   http://www.iquest.net/~msale
'69 Mustang 351W 5-spd (13.464 at 103MPH using cave-man technology).

All responses are my own and should not be mistaken
for those of Delco Electronics or General Motors.