[ROVERNET - UK] Long winded diatribe on profiling SU needles - maybe this will st op the tests.

Paul Smith Paul.Smith at auroraenergy.com.au
Mon Nov 22 22:29:30 GMT 2004

The SC was getting 23mpg, albeit with oversize tyres on the rear (195
something, they came with the P6B)(but they don't scrape the guards).
I leaned it out as far as I coul, it was nearly stalling under brakes -
better consumption, but not good enough.   Hmmm.
I consulted my SU book that lists all the carb specs for SUs fitted at
factories.  It highlighted that:
SCs always had the same piston spring
SCs had 4 different needles

There was a needle up to 65, then they richened the part where you drive
most up to 68, then leaned the whole thing bar the top 1/4" (which governs
idle up to mild throttle).
There was also a US emissions needle, which was between the last two needles
for mixture.
With older SUs (and Every SU I work on is older, never had the privilege of
a new one) there are air leaks along the butterfly shaft which lean the
mixture significantly at low airflows.  To enable the engine to idle you
wind up with the jet too low so it is rich across the range except idle.

1.  Fite pads on the shaft to block air leaks.  Haven't tried this one,
haven't had suitable material lying around.
2.  Drill the top of the throttle spindle bushings so you can drip oil in to
them - this lubricates them (less wear, yay) and the oil also stops some air
getting in.  Trick is to drill Just far enough to open a hole to the
spindle, don't drill the spindle much if you can avoid it.  Since the metal
is soft you can do it with a hand drill.  I used about a 1/8 bit.
3.  Richen the needle at idle.  Aha!
SU needles are measured every 1/8" with a micrometer.  This gives the
profile, measured at "stations".  Sites like teglerizer.com enable you to
find your needle profile.  All you need is the needle code, stamped (and
hardly readable) on the shank of the needle.  66 - 68 SCs used needle RN.
For some profund reason SU Always made needles the same for the first 2
stations - 99 and 95 thou respectively for a .1" jet (some carbs have .9" or
.125" jets). At idle the piston is close to right down - by my eye less than
1mm up on the SC.  This means the needle is about 98 thou at the venturi
bottom and the jet is a bit lower again.  
I mounted the needle in my power drill, shank in chuck, set up so that with
my rule butted to the chuck the 0" line was level with the shank end
(station 1, 99 thou).  I then marked off the 1/8" lengths on the needle and
measured the profile.  Note that you don't measure at the station with the
middle of the micrometer since needle is tapered.  My mic measured 98 and 94
thous, it is set 1 thou out (and since it was my uncle's I don't have the
manual on it to adjust it!).  I taped up the drill switch so that the drill
turned low - medium speed when powered on, and cut a 1.5mm wide strip of 600
grit wet & dry.
Power drill on, hold grit strip up to station 0, hold for 30 seconds,
remeasure.  It took about 2.5 minutes of sanding time to take it down 1

I refitted needle without changing jet position.  It was now too rich at
idle when up to temperature.  Good!  I moved the jet up 1.5 flats and the
idle is Lovely.  No flat spots, it runs Wonderfully.
I will monitor fuel consumption for the next 3 months and post it.

What would I do differently:  I'd say 1 thou reduction was tad too much,
would aim for .7 thou and see how it goes.  I also have a plan to fit an EGO
sensor to the manifold so I can monitor the mixture online...


More information about the rovernet mailing list