John,
I do a lot of work rebuilding and modifying SU carbs.
I suggest you customize your needles if you have access to an oxygen
analyzer and a small flat needle file
You'll also need a way to measure the "lift" of the dashpot at different
RPMs so you'll know where to file a flat spot on the needle.
Basically you run the engine up through the rpm band about 100-200 rpm at
a time and record the readings on your exhaust gas analyzer. You'll see
varying readings depending on the needle profile and how lean or rich the
mixture.
Record the dashpot lift at each incremental rpm.
Now comes the fun part:
remove the needles and coat them with machinists bluing so you can see
what you're doing on the next step.
Scribe lines around the needle at each incremental distance of dashpot
lift. The zero reference is where the needle meets the bottom of the
dashpot.
Now using your flat file you file a flat on the needles to increase the
annular area and the amount of gas that flows through the jet.
A wider flat equals a richer mixture. PATIENCE IS REQUIRED HERE WHILE YOU
GAIN EXPERIENCE LEARNING HOW MUCH TO FILE AWAY.
Clean the bluing off the needles, reassemble and repeat the process with
the analyzer.
I know this sounds like a major pain, but it is quite simple and becomes
quick and easy with a little practice.
Tommy
67 Sprite
60 MGA hot rod
Check out the new British Cars Forum:
http://www.team.net/the-local/tiki-view_forum.php?forumId=8
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