At 09:06 AM 5/13/97 -0700, Nolan Penney wrote:
>I'm finding myself somewhat befuddled in adjusting a Single Stromburg
>Carb. Silly me, I thought to myself, how hard can it be? I've adjusted
>So, here I am with a running Spitfire that wants to idle at around 1500
>rpm. I have determined that my vacuum retard on the distributor has a
>hole in it.
>Idle speed screw backed all the way out and not touching, no difference
>in idle speed.
>That air bleed screw thingie on the side, screw it out and eventually the
>engine just starts dying on its own. It will not simply lower rpm, it
>just keeps on going down until the engine stalls.
What year is the car BTW? It sounds like the you have adjusted the air
bypass valve. The strange looking unit on the side of the carb with a
triangular footprint is an air bypass valve. Its purpose (emissions and
drivability related) is to maintain a minimum pressure in the intake
manifold; if the pressure drops below a certain value, it opens and lets
air bypass the throttle plate.
If you haven't already taken the air bypass apart or adjusted it, leave it
alone! It is factory preset and you aren't supposed to mess with it. Your
problem is likely something else.
When you turn the bypass adjusting screw counterclockwise, it increases the
spring pressure on the valve. When you increase the pressure enough, the
valve closes. Since you have the throttle plate closed completely, your
engine is not able to idle in this condition. Hence your engine idles high
until you close the valve and then it slowly dies. Try the following:
Turn the adjustment screw counterclockwise about 5 or 10 turns. This
should completely disable the valve. Adjust throttle plate as described in
whatever carb tuning reference you are using and attempt to follow normal
procedure from there.
If this turns out to be your problem, then you are stuck with having to
iteratively find the correct valve adjustment. I don't have any great
suggestions on how to do this. Just try to find the minimum spring
pressure that will positively close the valve at idle.
>Choke, I have one, it worked the first time I started the car. Haven't
>noticed it doing anything since then. But then, I've lost track of how
>this thing works down inside that carb (where's the front butterfly?!).
>If it's an enrichner circuit, I don't know where it draws air from.
I think the choke control modifies the pressure ratio across the piston
which enriches the mixture. Others may correct me on this...
William Woodruff woodruff@fail.com
San Francisco, CA
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