Re: Vacuum switch

[Tom Harais <tjhemh@domain.elided>]

Mike:

That switch should not "make your engine die off the line". Not if you
are talking about the "High Temperature Vacuum Control Valve" that was
included with Scouts that had A/C and uses engine temperature to switch
the vacuum advance signal from ported to manifold.

That switch is considered part of the emissions control package,
although it's real function is to override part of the emissions design
if the engine overheats.

It has a manifold source of vacuum and a "ported" (no vacuum signal at
idle, but near full manifold signal otherwise - it's off a 'port' inside
the carb, right at the throttle plate level) source of vacuum. It
"sends" the ported signal to your distributor most of the time, which
means no vacuum advance at idle. But, if the engine begins to overheat,
that switch "switches" to the manifold source of vacuum which means full
vacuum advance at idle. This will up the idle RPMs slightly and help
cool the engine down.

Like I say, I don't know whay you mean by "broken" but it shouldn't
cause your engine to bog or die. I would bypass it for now, using the
carburetor ported vacuum signal straight to the vacuum advance or else
use the manifold signal straight to the vacuum advance. If your engine
is dying at part throttle, your problem lies elsewhere. If you think it
is vacuum advance related, have you checked the static timing and or the
action of the vacuum dashpot along with the mechanical advance
mechanism?

Tom H.