Vacuum advance & retard

[Tim Buja <buja@domain.elided>]

Mike Wattam wrote:
>However, as the vacuum advance makes very little difference to economy
>or emissions, my guess is this equipment has been stripped out as a
>waste of space.

However, vacuum retard will affect emissions at idle.  The retarded
spark will lower the idle speed, and allows the engine designer to
open the throttle butterflies a bit further to improve the air-fuel
metering.  Vacuum retard also improves engine braking on the overrun.

MkII Stags use a thermostatic vacuum switch mounted in the radiator
(mentioned in a previous post) in the vacuum circuit.  This "switch"
vents the distributor vacuum line to atmosphere as it closes off the
line to the ported vacuum source at the carburetor when the coolant
temperature is above its setpoint.  This advances the spark, increasing
the idle speed and coolant flow in an attempt to reduce the coolant
temperature.  Does anyone know of a source for this vacuum switch,
which appears to be unavailable?

> Related issue 1 - check whether the vacuum works by sucking on a
> connected hose - you will probably also find the diaphragm punctured.
>
>Related issue 2 - remove the distributor cap and see if there is free
>play between the distributor base plate and the vacuum pushrod.  If
>so, the small screw which locates these together is worn out.  Fitting
>a new plate assembly (with included screw) will give a much better
>tickover and possibly slightly improved performance.

I had my vacuum capsule (retard only) repaired by M&G Vintage Auto Co.
in Tuxedo Park, New York, for $65 two years ago.  (Their phone # is
(914) 753-5900.)  At the same time, I ordered a new breaker point
mounting plate assembly from Rimmer Brothers, since my plate was
significantly worn where the three tabs hold the upper, moving, plate
to the stationary baseplate that is screwed to the distributor body.
The new plate assembly DID NOT include a new screw.  In addition, the
hole for the condenser was drilled but not tapped.  I notified Rimmer's
of the faulty part but they did not offer to replace it.  I wrote the 
SOC seeking a source for the pin, but they only stated that it should
have been included with the baseplate and doesn't appear to be
available separately.

As a side note, does anyone know where can you purchase the screw?
Mine is quite worn where it engages the vacuum capsule's actuating
arm.  My neighbor works for a local screw manufacturer, so I gave it
to him to see if they made anything that could be modified for this
application.  He measured the threaded portion of this pin to be 0.183"
diameter, 62 threads per inch, and couldn't find anything similar in
any of their products.  He suggested that I contact a machine shop...

Replacing these part made a distinct difference in the tickover
quality.  Unfortunately, I found that the diaphragm was punctured
again when I tore the engine down this summer, so I ordered
another unit since hole in the actuating arm that engages the pin
to rotate the upper plate was quite worn.  You can see the
difference between the old and new vacuum capsules on the VTR Stag
maintenance page at

http://www.vtr.org/Stag/Stag-maint.html

This page also has pictures of the proper reverse poppet valve
thermostat that is required to block off the coolant bypass flow
path at operating temperature.  The photos also shows a standard
thermostat that will not block this flow path, causing the engine
to overheat and boilover after 25 miles at 65mph.  I found this
the hard way...

Tim Buja - Rockford, Illinois, USA - 80 TR8, 73 Stag, 72 TR6