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

[Jonathan Hewison]

test 


Jonathan Hewison
Engineering Technician
Faculty of Architecture & Design 
Victoria University of Wellington
Te Whare Wananga o Te Upoko o Te Ika 
New Zealand. 
www.vuw.ac.nz
-----Original Message-----
From: atkin [mailto:John at atkin4174.freeserve.co.uk] 
Sent: Tuesday, 23 November 2004 12:00 p.m.
To: rovernet at lyris.ccdata.com
Subject: Re: [ROVERNET - UK] Long winded diatribe on profiling SU
needles -maybethis will st op the tests.

Hi Paul,
I seem to remember being able to get new bushes for the butterfly
spindle, also rep,acement spinddles.
best wishes
JOHN atkin
----- Original Message -----
From: "Paul Smith" <Paul.Smith at auroraenergy.com.au>
To: "Rovernet (E-mail)" <rovernet at lyris.ccdata.com>
Sent: Monday, November 22, 2004 10:29 PM
Subject: [ROVERNET - UK] Long winded diatribe on profiling SU needles -
maybethis will st op the tests.


> 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.
>
> Solutions:
> 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 thou.
>
> 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...
>
> PVS
>
>
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