On 10 Jun 2007 at 21:36, Scott Suhring wrote:
> could you possibly explain what the thermostatic
> "switch" is supposed to do when hot?
Wow. I don't have a '74 TR6 so I sure couldn't follow all that
description. But I'll throw out one idea I heard long, long ago.
Does that dizzy have dual points? If so, I'd guess the following.
They are intended to be set differently, one opening at the normal
timing, the other at a special timing, probably earlier (at least at
idle) and meant to apply only during cold startup when the emission
regs might not be so tight. They would be wired in parallel, and the
later-firing points would be in series with the thermoswitch. The
thermoswitch woulde normally be closed when the engine is hot, open
when it is cold.
It works like this. Since the plugs fire only when the circuit is
cut, the opening of the later of the two determines when it fires.
As long as the engine is cold, the late-firing plugs don't apply
because their circuit is always open. When the enging reaches a
certain temp that switch closes, which keeps the current flowing in
the coil's primary winding until the later points open.
Now, I could have it backwards about whether the earlier firing is
for hot running or cold. The thermoswitch has to be on the later of
the two points (otherwise it would have no effect), but the switch
operation would be reversed, closed when cold and open when hot.
You see, there is a second consideration. You want optimal timing
when running at speed under power. The emissions target back in '74
would have been for idle, not at speed. A timing set really late for
clean-idle purposes has to be advanced back to "normal" at speed. So
they would use a vacuum retard to pull the timing back at idle, and
that retard would go away when the throttle was cracked open. I'd
guess that they had trouble with an arrangement like that not running
well when cold. So they used a second set of points that didn't have
the retard. This set worked when the enging was cold, but once it
warmed up the second, retarded-at-idle set kicked in. The only real
issue left to solve would be to make sure that it ran well at speed,
and of course the at-speed-when-warm timing will be the later of the
two. So if they spec'ed the vacuum retard to be the same as the
amount the early points were to open before the later ones, then the
two would open more or less simultaneously.
As I said, I'm guessing. Maybe it doesn't have dual points at all,
and the thermoswitch is related to an air/gas mixture valve.
Now where did I hear about dual points? My Datsun 510??
--
Jim Muller
jimmuller@rcn.com
'80 Spitfire, '70 GT6+
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