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

Slatskars slatskars at comcast.net
Tue Nov 23 01:47:06 GMT 2004

Over the next three months, I will be working on a couple of ideas to cure 
the air leak problem. One is to attempt to fit HIF type seals to H and HS 
type carbs. The other will be to machine "V" groves in the shafts to take 
"O" rings to seal the leak. Looking for a source of Viton or Silicon "O" 
rings of the proper size. This has been on the back burner for a while, but 
I am being pushed a little for a workshop at the end of Feb.

Will let the list know the results.

Vancouver, WA, USA
----- 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 2:29 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 
> sensor to the manifold so I can monitor the mixture online...
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