I just finished building up mild LA360 for
my ’69 Dart. I tried my best to copy the
specs of the old Mopar Performance
360HP crate motor. I used 9.5:1 hypereutectic
pistons, and the same 284°/0.484˝
cam. I had a pair of ’70 340 X heads
mildly ported and we installed hardened
exhaust seats. The carb is a 750 Holley
vac. Secondary.
Since this is my 3-season driver, fuel
economy matters, especial as premium
is now back over $3.00. I used a vacuum
electronic distributor, which I hooked up
to the Holley’s ported spark fitting. Problem.
If I leave the vacuum advance connected,
the car pings at light throttle. It
also has what seems like a “lean surge,”
though I can’t imagine the Holley could
be lean! The vacuum advance seems to
really help mileage, so I don’t want to
give it up, but, with it connected, I have to
retard the initial timing too much that the
car really slows down.
How do I fix this?
You just need to do a little “tuning.”
Remember that, even back in the “pre-emissions”
good ol’ days, the factory
engineers spent quite a bit of time dialing
in the spark and fuel “curves” for
each powertrain combo. Its unrealistic to
assume that you can just slam together a
slew of pieced-from-the-catalog parts and
have everything just “drop into place.”
Not gonna happen.
You basically know what’s wrong:
too much spark lead when the engine’s
lightly loaded. The cure? Just retard the
timing under those conditions. How to
do it? Tweak the vacuum canister on the
distributor.
Two parts to this tweak: First, insert an
Allen wrench through the hose nipple,
and rotate it counterclockwise, see photo
. This increases the spring tension
on the diaphragm, and delays the
onset of vacuum advance until there is
more vacuum. Try two turns, or more, as
necessary, until the knocking and surging
are gone.
If you have to go more than 3 or 4
turns, there might just be too much
vacuum advance, rather than too soon.
The fix here is to either swap to a canister
with less advance (the number is
stamped on the arm,) or weld up the
stop on your present arm.
If you get real lucky, you might find
a Carter “Engine Knock Eliminator”
bouncing around on eBay. This uses an
intake-manifold mounted microphone to
“listen” for knocking and retard the timing
as required, an early incarnation of
what’s on virtually every modern car.
Remember: any detonation or knocking
you can hear is very bad and very
damaging.-
