>At 05:39 AM 6/16/99 -0700, john donohoe wrote:
>>Can you explain why blanking the bypass valves helps the high idle
>>problem, and also suggest what the *real* problem might be? Blanking
>>the valves works great for me, but it seems to be a "work-around" and I
>>would much prefer to solve the root problem!
>> And At 08:14 PM 6/16/99 -0400, Sam Gentry wrote:
>
>The bypass valve "bypasses" the throttle plate. If the valve is open all
>the time then in effect the throttle will not close. Blanking the bypass
>"closes" the throttle. I have had some success cleaning and reinstalling
>the bypass.
And Atwell once again shows that he has read the Bentley manual (a copy
resides in the home 'library') by explaining the theory:
The bypass/deceleration valve does what Sam said: it provides an opening
bypassing the throttle plate, so that when the throttle is snapped shut an
overly rich "pulse" of air/fuel mixture is not sent to the engine. (Like
holding the throttle open a bit) It was fitted as an emissions device.
(Other carbs have used dashpots on the throttle to achieve the same effect.)
Inside are some air holes, a diapragm, and IIRC a spring. Apparently when
the spring weakens it no longer holds the diapragm shut so the idle speed
increases.
The Bentley manual provides an adjustment procedure, but it cautions that
if the b/d valve cannot be set to spec it must be replaced.
Despite this, VB and SpitBits have rebuild kits. I haven't tried them...
Atwell "well-read" Haines
'79 Spitfire
Succasunna, NJ USA
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