Ignition timing set-up

[Tom Cloud]

>> 
>> 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.

personally, I'd not worry too much about the EGO reading,
except to be sure you're not running too lean

>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.

I've taken my distributor and silver-soldered part of
the slot the mech advance tab moves in to limit the
max centrifugal advance and then turned back the vacuum
advance .... and that seemed to work (I broke that distributor
putting the steel cam gear on it so I'm back to square
one).

Problem with that method is there's this guy with torch
in hand blobbing silver solder on until he says "yep, looks
about right".  Then he files off the excess.  Problem
with this problem is this guy likes to be a little more
precise than that.

>From my observations, I "think" that I want about 18 - 20
degrees initial advance (you're right about the starter, though)
with mechanical advance limited to 10 degrees max and maybe
another 5 degrees of vacuum advance

But .... is that right ??  I'll tell you it *feels* good !!

And then, where to hook the vac advance ??   With ported vac,
the extra 5 degrees would come in as the throttle is tipped
in (is it in at part throttle cruise ???? if so, that would
answer my question -- that'd be the place to pick off the vac).
If the ported vac doesn't affect part-throttle cruise, then
seems manifold vac would be the best ????   oh, myyyy ....

and the efi content here is that I've converted to efi (though
it's not diy)

Tom Cloud

        He laughs last thinks slowest