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Weber downdraft Re: SU Needle difference

To: Lancer7676@aol.com, eriks@netspace.net.au, pasgeirsson@juno.com, spridgets@Autox.Team.Net
Subject: Weber downdraft Re: SU Needle difference
From: Daniel1312@aol.com
Date: Fri, 11 Dec 1998 16:05:01 EST
Reply-to: Daniel1312@aol.com
Sender: owner-spridgets@Autox.Team.Net
In a message dated 11/12/1998  11:43:30AM,  Lancer7676@aol.com writes:

<< but since I have a downdraft Weber carb on my 1500,
 and it acts real quirky, maybe the needle selection is something I need to
 look at?  Do these come in sets or do you have to know the size you are
after?
 If you have to know that size--How do you know?  And is this the needle you
 adjust when you adjust the mixture on the carb?  Just unscrew and take it out
 and replace it with one of these new ones?
  >>
The downdraft Weber is a very different type of carb to the SU.  Jetting
changes are made by change of jetting size rather than needle size.  Bigger
jets have bigger 'holes' in them that let in more fuel.  The Weber will have
several jets unlike the SU's one.  I have heard that British Car Magazine are
due to run a feature on DCOE Weber carbs soon which will aid your
understanding even though the DCOE is a sidedraught.

The only proper way to set up the carb mixture is on the rolling road/chassis
dynometer.  For reasons that I have yet to fathom, NO-ONE in the entire USA
spridget world seems to use one.  I do know that they are scarce in Australia.

The alternative way to check mixture strength requires considerable skill but
if you know anyone who is a drag racer they will know someone who has the
skill.  The skill in question is to do a 'plug cut' and 'plug read'.  If I
remember correctly you drive up a hill with the engine under load and heavy
throttle and then dip the clutch while simultaneously switching off the
ignition.  You then remove the plugs and say what colour they are.  I think
Vizard's book contains  a better description.  Depending on the colour of the
deposits on the plug the mixture at the point in the rev range is either ok,
weak/lean or rich.  You experiment with jet changes till it is right.  All of
this takes no time at all on the rolling road and you see the exhaust gas
analysed on a scope as well as changes to the power reading the engine
produces.

Daniel1312

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