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needle selection

To: <fot@autox.team.net>
Subject: needle selection
From: "Aaron Johnson" <fpspitfire37@msn.com>
Date: Tue, 10 Feb 2004 23:18:32 -0800
Okay...next carb question.  I've got the 1.5" su's all cleaned up.  I
decided to put off modifying the bore and/or thru boring for a year
until I can have time to fully research and enquire on what needs to
happen to make it work right.  
 
What I had was a comp ratio of 12:1, using an A-8 cam, 1.5:1 roller
rockers and 1.25" carbs with a GT (maybe BB3) needle and .090" jets.  It
seemed to be just about right at all rpm range at sea level.  
 
What I'm switching to is a comp ratio of 14:1, using a cam with .560
lift, 310 duration (with 1.5:1 roller rockers) and 1.5" carburettors
with thinned throttle shafts and the flash removed. 
 
I'm waffling back and forth between whether to use .100" jets or .090"
jets, and I'm not really sure where to start with needles.  I've heard
what might work is somewhere in the 3, 4 & 5 range of needle with .090
and KW with .100.  These are fixed needle sizes and I've tried some
comparison with spring type needles (which I'll use this year and then
switch next year) and came out with BBA, BBZ or BCH.  
 
Are there pro's or cons to using one jet size over the other?  
 
Any suggestions out there for where to start.  I know the true answer is
to throw the thing on the dyno and play with it until I find the perfect
setup and have a couple similar needles for weather conditions, but
alas, I can barely afford to eat at the race track let alone buy an hour
of dyno time.  Ask Bill B. he's seen what I show up to the track with :)
Is there a good starting point, I didn't see anything in the competition
prep manual.
 
All the best
 
Aaron Johnson
#38 F Production Spitfire Mk4 Oregon Region SCCA
http://www.geocities.com/spitracer9

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