A few things to understand about SU's.
The metering device on an SU is the tapered needle and the jet - the hole
that the needle goes into at the bottom of the carb. Mixture while running
above idle is adjusted by changing the needle for one with a different
taper. In almost all cases the jet stays the same size. What the tapered
needle does is vary the usable cross-section of the jet depending on how
far out of the jet it is based on how high the piston has been pulled by
the engine vacuum above the piston.
Adjusting the mixture at idle is accomplished by raising or lowering the
jet in the body of the carb - this assumes that the weight of the piston,
the spring, etc, have cause the piston to be at or near the bottom of it's
range of movement. since lean indicates less gas, while rich means more
gas when compared to air, you can see that lowering the jet in the body
(screwing it out, or counter clockwise) drops the jet onto a narrower part
of the needle, so making the usable cross-section larger, and allowing more
gas - richer. Raising the jet means smaller cross-section, so leaner.
The idle mixture is right when you have obtained a point where raising the
piston very slightly (1/16") at idle causes the idle speed to go very
slightly up, then fall back to where it was, or stay about the same. If
the speed goes up, then you are too rich, if it goes down then you are too
lean. This only really works on a car with a mild street cam - race cams
barely run at idle, so fine tuning is rather moot.
Now for an area of confusion. When you talk about clockwise and
counterclockwise with regard to a screw, you are referencing the screw from
the "top" - to the screw! imagine screwing a wood screw into the top of
your work bench - you are turning it clockwise. Just because the jet
adjustment screws on an SU are upside-down when installed on the car,
doesn't mean that you don't still screw them clockwise to get them farther
in to the body of the carb!
All this does is set the idle mixture, same as all balancing the carbs does
is set the idle balance. As soon as you open the throttle, the balance is
set by the pistons reacting to the engine vacuum, and the mixture is set by
the needles.
Cheers (this is why I run Webers) brian
At 01:06 PM 03/21/2000 -0500, thom kuby wrote:
>Greetings trumpet players,
>I have a question regarding fine tuning these SUs...on a TR3A - I'm
>assuming they're the stock units.
>
>ok... I THINK I have this right.
>
>In order to completely lean the carbs out - you turn the nut (on the base
>of each carb) completely counterclockwise (to close 'em)?
>and then you turn clockwise, a "flat at a time", until you get the right
>mix? is this right?
>
>I'm using a Redline Synchrometer for balancing...right now I'm getting a
>reading of approx 7.5 on both carbs @ about 700rpm
>
>It doesn't sound bad at all...but I'd just like to know what the proper
>fuel mix is on these critters. Since my experience with SUs is limited to
>just this one car (so far) anyone care to walk me through this process? I'd
>really appreciate this.
>
>comments anyone?
>
>Thanks...I appreciate the time. oh, please do me a favor and respond
>offline (as well as to the list - it's just that I don't always get around
>to reading the digests, that's all...)
>
>Cheers
>Thom Kuby
>Porsche freak, with a TR3 in his garage
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