SU again

[saabfast]

Probably a stupid question, but the manual says that the shoulder on the needle (in the carb piston not the float chamber) should be flush with the underside of the piston, but does this mean the needle itself or the needle carrier/guide?
Mine had the carrier flush which meant that the brass needle shoulder was slightly protruding. To make the needle flush means that the carrier is slightly recessed.
Maybe pedantic, but I'm getting tired of taking them apart.

Alan
Saab 9000 Stg 1
'81 TR7 DHC

[Hasbeen]

Alan, if the needle is TOO far out, the shoulder could hit the top of
the main jet, which could
1/ damage it slightly, which could effect its metering capacity. This is Unlikely.

2/ prevent the piston shutting, holding it a little open, making it
difficult to ballance the carbs.

A visual check will show if there is a problem.

If it was too far in, it would make the needle effectively slightly
richer. Too far out, slightly leaner.

Any change to the mixture by this, would be corrected by adjusting
the main jet one or two flats of the mixture adjusting nut, on that
carb.

Hasbeen

[saabfast]

That is what concerned me Hasbeen. It was originally set so that the carrier was flush with the bottom of the piston and therefore the needle (which is sprung and is not adjustable) was slightly protruding (about 0.5 to 1mm). This could possibly hit the jet, although I don't think it would as it is generally lower than that. I have reset it so that the needle shoulder is flush with the the base of the piston (hence the carrier slightly recessed), but thought that might be making it too rich. Setting of the jet might alleviate this but would it not mean that for a give airflow anywhere in the range it was running richer?
I have set both carbs the same.

Alan
Saab 9000 Stg 1
'81 TR7 DHC

[Hasbeen]

Alan, what controls the amount of fuel going into youe engine, for a
given velocity of air through the venturi of your carb, is the
position of the needle in the main jet. If you take the area of the
hole in the main jet, & reduce it by the cross-sectional area of the
needle, at the top of the jet, you get the area available for fuel
to flow through, onto the air stream. As this tapered needle is
raised, its area in the jet is reduced, increasing the area for fuel
to flow through.

You could raise or lower the combination of the pair, a couple of
mm, & provided they did not protrude into the air flow enough to
cause serious turbulence, it would not effect the mixture, as the
area for the fuel flow would be unchanged. If you have raised the
needle a little, you will have to raise the main jet a similar
amount to achieve the same fuel air mixture.

When we tune the idle of our cars, it is most important to get the
air flow through both carbs as close as possible to identical. If
everything is in good order, this will allow us to adjust the main
jet to get the correct amount of fuel into each air stream, & thus
into each cylinder, giving a nice smooth idle.

Once this relationship between the needle & jet is set at idle, the
mixture through the rev range is then controlled by the taper of the
needle. This is why we play with differently tapered needles when we
modify our cars, to achieve greater performance.

Hasbeen

[saabfast]

Thanks Hasbeen, but I appreciate that. It was why I was trying to tie down the definition of setting the 'shoulder of the needle flush with the bottom of the piston'. It would make a slight difference to the fuel orifice, even if only 1mm difference in needle position.

Alan
Saab 9000 Stg 1
'81 TR7 DHC

[Hasbeen]

It will only make a difference until you tune the main jet.

You adjust the position of the main jet, to get the mixture correct,
as part of the tuning process.

You do this by screwing the main jet nut up or down, until lifting
the piston by a 1/16" or so gives a slight increase in revs. This
corrects for variations in float level, [height of fuel in the jet]
& slight differences in needle height, & variations in the
performance [suck] of each cylinder.

Hasbeen

[flynns tr 7]

The brass shoulder which holds the needle in place should be level with the piston.Make sure you have the groove in the shoulder in line with the screw when you tighten the screw or you will damage the brass shoulder.