On 26 Dec 2007 at 17:04, tom white wrote:
> Setting the points determines how long a plug fires.
Not be a nitpicking scrub, but this just slightly misses the point
(no pun intended :-). Plugs start to fire the moment the points open
and, yes the firing does require a small time interval. But the
important issue is the time spent closed, not the time spent open.
The time spent closed is the critical period for coil current to
build up for the next firing. During the firing portion, as long as
there is enough time at all, once the plug has fired any extra time
they spend open is of no consequence. After that the points need to
be closed only long enough to build up the perfect current; any
longer simply heats up the coil. At high rpms you could run out of
time between firings, and the most likely problem is running out of
current-build-up time, not actual firing time. The point being, you
aren't really adjusting the time for firing, you're adjusting the
time for getting ready for the next firing.
Consider: At 6000rpm a 4-cylinder has 12,000 firings per minute, for
5msec per firing. Suppose the dwell duty cycle (i.e. the percentage
of time spent closed) is 50%. This means 2.5msec open, 2.5msec
closed. By comparison, 2000rpm allows 15msec between firings. But
if 2.5msec is long enough for 6000rpm, it should be long enough for
2000rpm too. So at 2000rpm the dizzy could be open for 12.5msec, and
closed for 2.5msec. In theory this would keep the coil cooler.
(I've never had a coil go bad [** knock on wood dashboard] but some
of you folks apparently have.) Of course, dizzies don't work that
way. If it was set to 50% then the closed and open times would both
be 7.5msec.
When "setting the points", one typically sets dwell by adjusting the
maximum gap to which the points open. If the cam lobes are symmetric
(and the points don't bounce too much), both the opening time on one
side of the lobe and the closing time on the other side are affected,
both moving either closer to or further from the max-open point.
That's why you have to set the timing last. You have to restore the
proper opening time which got changed when you adjusted the point
gap.
Happy New Year, everyone!
--
Jim Muller
jimmuller@rcn.com
'80 Spitfire, '70 GT6+
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