[Healeys] HD8 Slow Run Valve

[Alan Seigrist]

Bob -

Controlling idle speed with the screw is very simple - it is a narrow
valve that is highly stable, controlled and consistent.

Using the throttle butterfly, you have issues of throttle linkage,
dissimilar metals (brass, aluminum & steel) and fast idle screw
wobble.

On my BJ8 once I set my rebuilt carbs to factory spec and idle through
the slow idle port (and fully shut throttle plate) it has maintained a
perfect and constant 550 rpm idle for over 40K miles without so much
as a touch from my screwdriver.  Of course I put in a pertronix kit
which made a huge difference too.

Strangely enough, the carb failed because the jet diaphragm finally
went, but I didn't touch the carbs for more than 5 years of almost
daily running.

The carbs are very well designed if set up factory spec.  The little O
rings on the slow idle screw are the key to the success of a proper
tune.

Alan

On 7/19/10, Bob Spidell <bspidell at comcast.net> wrote:
> Alan,
>
> Temperature, humidity and the local barometric pressure affect air
> density, and overall engine performance and behavior.  How does the
> engine know whether the mixture came through the slow run valves or the
> throttle plates?
>
> My BJ8's idle varies depending on the aforementioned conditions, but
> only slightly unless I get a few thousand feet above my usual sea level
> conditions, or it is exceptionally hot (lower air density).  It is
> always steady and smooth.
>
> bs
>
>
> Alan Seigrist wrote:
>> On HD8s, cracking the throttle plates open works ok but bypasses the
>> purpose of the slow run valve, which is to give you a nice steady &
>> smooth idle.  If you crack the throttle plates open, you'll have a
>> couple hundred RPM variation depending on temp / conditions, etc.
>> Incidentally on a car with triple HD8s like an S1 E-Type you'll never
>> get idle below 800 or so if you crack open the plates.
>>
> *******************************************************************
> Bob Spidell           San Jose, CA            bspidell at comcast.net
>
> *******************************************************************
>
>

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