>You may be correct about the advance. I have a vacumn
>advance that has a mind of it's own. The pinging occurs
>when I go from light load to heavy load. The vacumn advance
>has the timing advanced just before that point.
The vacuum advance should drop out of the equation when you
go to wide open throttle. If it's just a transient problem, then it may
well be the vacuum advance mechanism. Mallory used to make a
Speed-o-Lite dual point kit which got around the transient problem.
They did away with the vacuum advance mechanical parts entirely,
replacing them with a vacuum referenced switch that switches
between dual point and leading point operation. This provides a
pseudo vacuum advance that operates instantaneously. It also
does away with the high rpm instability that can be associated with
vacuum plates.
Are you running the stock Ford distributor? If so, they made them with
two different sets of mechanical advance plates. The most common plates
are set up to limit mechanical advance at either 13 or 18 distributor
degrees, depending on how they are installed. The ones set up for 10 or
15 degrees are better for quench chamber heads. Install them so mechanical
advance is limited to 10 distributor degrees (20 degrees at the crankshaft).
Clevelands with closed chamber heads usually make best power at 32 to 36
degrees total advance, so your initial should be between 12 and 16 degrees.
In my Pantera, I'm running a Mallory Unilite distributor with adjustable
stops for the mechanical advance. I set mine at 18 crank degrees but the
supplied springs limits it to 16 degrees on my timing light. Mallory has
a chart and a procedure to help you pick the proper springs based upon the
limit setting but they don't provide that information unless you call them
up.
> I had MAJOR problems 2-3 years ago with
>backfiring and missing when going from low load to heavy
>load and even on steady driving.
Another common 351C distributor related problem that can cause
similar symptons is the gear pin. If debris causes the oil pump
to momentarily jam, the pin is the weak point and usually shears.
The gear may rotate around, then jam in place, dramatically
changing the timing. It's good preventative maintenance to double
up the pin or replace witha tool steel pin.
> I seem to recall that I took extra care in computing
>the compression ratio. The stock pop up pistons were only
>11.5 to 1 and they had a pretty good pop up.
Ford listed the Boss 351 (quench heads and pop ups) at 11.7:1,
the flat top/quench combo at 10.5:1, the flat top/open chamber
combo at 9.2:1 and the open chamber/dish piston at 8.6:1.
Aftermarket pistons are often designed with the less deck clearance
so may have slightly more compression.
Dan Jones
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