>From my mail- I seem to have opened a rat's nest.
Most of us are totaly naive when it comes to "tuning" SU carbs and their
potential. I have been playing with Britmobiles for 40 years and just realized
that I had NO idea of how to "tune" an SU untill a year ago!
I saw a couple of guys with a GP car swapping needles at a race- I asked them
what logic they used to determine their change. "Oh, we just had some extra
needles and thought we'd see if it ran any better". I gotta admit- that was my
approach too, for years....
Did you know that there are 354 needles listed for your .090 jet SU! And....
your car will probably run on 80% of them? Why so many needles?
When you "tune" SUs according to the book- ALL you are doing is synching them
and setting the idle. You are "trusting" that the needle is right at all the
rest of your throttle/load conditions. Sure it may run "fine" however you are
probably not spot on as far as mixture is concerned, except at idle! The real
fact is that you may be WAY off and never know it! Engineers set the needle
specs from the factory and you live with it.
However- if you really want to get the most from your engine- and don't like
the idea of washing the rings with a rich mixture or burning the pistons with
lean- or just want a crisp, responsive engine- you gotta "tune" those SU's
for your particular setup. This will more than likely involve a change of
needles. Other's recommendations, even "factory" recommendations are probably
off somewhere in your driving range... maybe WAY off! Just remember "different
is not the same". Different car (and EVERY one is different) - different
requirements- different needles....
There are a jillion factors affecting mixture- exhaust system- aircleaners-
engine condition- altitude- CR- load- RPM etc, etc...
The ONLY way to tune an SU is with an exhaust analizer ($100 bucks). Anything
else is groping in the dark.
.
SU needles each have a number/letter designation which is of little value
unless you know just how the needle is contoured. There are really very few
needle designations among the 300+ that are "progressive" in any way. You
gotta consult the chart & compare numbers. Smaller diameter is richer and
there are at least 12 stations per needle! There is a lot of tuning potential
there.
The full SU needle chart shows the diameter of all the 354 needles at 1/8"
intervals. These are called "stations". Stations 1-2 are basicaly idle and are
the same for most of the needles. As you progress up the needle, you are
affecting off-idle, low mid-range, high mid-range & WOT (wide open throttle)
My car ran GREAT. No problems- drove it daily & I was completely satisified
the it was spot on. Then one day I checked it with the K&N analizer- it was
real lean when accelerating! I never would have guessed....
NO "adjustment" will cure that. If I richen up the carb (drop the jet a few
flats)- I could get more acceleration- noticible by the seat-of-the pants
too!! However- now idle and mid-range would be very rich.
Then you gotta look at the needle chart for one that is the same up to say,
station 5- and then progresses to a smaller diameter. This will affect only
acceleration. Then you gotta try it and check it with the analizer. It is
amazing just how much difference you can affect in your engine's performance
by getting the mixture right.
Example:
AH 890 862 830 803 775 756 733 711 690 670 650 630 610
AH1 890 860 820 790 765 750 730 710 690 670 650 630 610
(AHI is slightly richer in mid-range)
AAM 890 850 824 785 752 715 673 630 590 567 543 519 496
(compared to AH- would be much richer- off idle on...)
AI 890 850 817 798 780 765 750 732 712 693 685 675
Compared to AH- would be richer low-mid range progressing to leaner high-
range)
Somewhare in those 354 needles is one perfect for your car! We won't reach
perfection, but I guarantee you that you can get damn close and you will be
amazed at the difference!
There is a DOS based program called "Haystack" that lists all the needles and
helps you to find the next logical needle by searching specific stations for
you. That, combined with an analizer can keep you busy playing for months..
SU carbs have a LOT of potential. When someone says that they can't tune their
SU's- I guarantee you that they mean that they have set the idle and simply
"trusted" that the needle was right- and it wasn't.....
Fred Sisson
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