Trevor,
RE:"I do not even know what the bypass valve bypasses."
The bypass valve found on most CD2-SE Zenith-Strombergs
is meant to bypass the throttle plate when it is closed and the engine is
on what the Brits call "overrun" and what I suspect many of us in the US
might think of as "engine braking". When manifold vacuum ("depression")
gets too high, the valve allows a small amount of air/fuel mixture past
the closed throttle plate, thus limiting the vacuum and consequently, the
high emissions produced in this situation.
The post-'68 US-spec SU
carbs achieved this effect with a comical poppet valve device directly on
the throttle butterfly. At wide open throttle, this valve blocked
considerable
airflow into the engine. It also presented a damage hazard if it ever came
loose during operation.
Good luck with your TVS riddle.
Best Regards,
Tom Marincic
|