Marcus:
Glad that seems to have gotten it.
However, I'm in a weird mood, and feel like explaining what was going on.
The thumb wheel on the advance can does not restrict movement in any way.
It just sets the position of the can in relation to the distributor body.
And since it's directly connected to the breaker plate and thereby points,
it sets the point position relative to the distributor body.
I talked a lot about distributor phasing at you. I neglected to explain
exactly what it is. First off, you must understand that there are no
absolutes in the engine timing. Everything is relative. As a given piston
comes up on it's turn to fire, The distributor is supposed to make a spark,
and get it to that particular cylinder. Heres where the relatives come in.
From this piston, via rod, crank, and cam, you have the distributor shaft.
This shaft has to elements; one is the cam that opens the points, the other
is the rotor. These two are hard connected, and can not move in relation
to each other, ever.
Now, when the shaft comes to the point of firing, you need two things.
First, you need the igntion points positioned such that cam lobe opens
them. You also need the distributor cap positioned so that the correct
wire lug is offered up to the rotor. This is all covered by the distributor
body. It holds the points, and the cap. Both are attached to the body in
a fixed geometry. When the points get bumped open, the rotor is sure to
be pointing at one of the lugs. When you turn the dizzy to set the
timing, you move the points and cap by the same amount. The geometry is
maintained.
On the other side, you have the centrifugal advance. This advances the
cam lobes in relation to the rest of the engine, advancing the timing. And,
the rotor is firmly attached to the cam lobe, so it also advances. Again,
geometry is maintained.
Neat theory, and it worked well until the advent of vacuum advances and
aftermarket points and electronic conversions.
The problem is... the point position is not a fixed constant relative
to the distributor body. Different brands of points have the rubbing block
in slightly different positions; moving the effective point position. Also
The vacuum advances job is to move the points (via the pivoting breaker plate)
in relation to the dizzy body. This is why the rotor has such a wide arm
at it's tip. It's this way so that it always offers up a portion to the
cap lug, no matter where the points are with the designed bounds.
In your particular case, you may have points with the block slightly off.
The rotors currently available do not have as wide an arm as the old ones.
Combine this with your worn breaker plate, and the thumb wheel adjuster
being wound all the way in (moving the plate/points way over yonder), and
it's very possible that under certainconditions the rotor was pointing off
into space when the points fired. Imagine having a 40 or 60 thou air gap inside
the dizzy for the spark to jump before thinking about crossing the plug gap.
And the direction yours was off would cause this to happen at high load (when
the advance can was out), when the most spark is needed. Misfire city.
TR6's are much more prone to this, as they run a retard can, rather than
advance. This moves the points in the other direction, and the new crop
of rotors don't take this into account. The trick here is to move the
vacuum can until things line up under all conditions. Trial and error, but
I usually end up moving the critter about a 1/4" out.
But the most famous culprit, the one that made me puzzle all this relative
logic out in the first place, was the old Allison electronic conversions
for the 45D4. These things came with universal triggers with long slotted
mounting brackets. Invariably, the installer put it in so that the wiring
laid out all pretty. Like this, when the trigger fired, the rotor was
about 30 degrees from the nearest distributor lug. And then the carb would
be blamed for the crappy running.
Allison was long since cured this by developing custom fit pickups; at
least for the 45D4's. But the bad taste left from the first ten years of
the universals is still with me to this day.
Randy
randy@taylor.wyvern.com
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