I got this note from Julian- some might find this interesting...
Fred Sisson
from Julian:
I would like to try the air/fuel ratio setup to help me out, but have a
question. Basically I've talked with some people, including a few who have
tried it on various applications, and they say that the O2 sensor works on
an EFI system because the mixture is pretty close already...but that on a
carb system you would get a full-lean or full-rich reading 99% of the time.
So, will an air/fuel ratio gauge work well in tuning a set of SU's that
may not be particularly close to 14.7:1?
My reply:
Julian-
Your friends are right- the meter jumps all over the place while driving on an
engine with SU carbs and I would say that the mixture will be way off 50% of
the time when driving. Don't let that bother you at all! It is the nature of
the beast..
Basically what you want is for the mixture to be close on smoothe cruise and
to read slightly rich (but not off the gage) on hard acceleration- and to be
on-the-gage in mild acceleration. On-the-meter idle is nice too- but not going
to hurt anything if it is somewhat rich or lean, as long as it idles. If idle
is too rich- it will start in an instant (even cold) but prolonged idle will
soot plugs- too lean & it will crackle & may only start with choke.
Yes- the meter is sensitive and blanks out quite a bit- for instance when you
move the pedal the least bit when cruising- there are a few moments when
mixture will be way off and then will come back on again as the carb adjusts-
no problem. Also it will go full lean (or rich- I forgot) upon deceleration-
again, no problem.
Set your priorities and work from there..
Mine are:
1. Hard acceleration- WOT (wide open throttle)- I want max power & don't want
to hole a piston. I go for around 12:1. I have a lot of time & money in that
little engine & don't want to damage it...
2. Cruise & mid range- affects daily driving- get it close to mid-meter (14:1
or so..) and the car will be ever-so-much "more responsive".. (check cruise at
around 50 mph- high gear & steady)
3. Idle- get it on-the-meter so the plugs won't soot up & car will start
easily & idle nice
That's it. Don't get nuts over this- but aim for these three ranges. You will
be absolute delighted at how much difference the proper needle for YOUR car
can make.
Here is a "real world" example..
I just re-needled my SU's yesterday with meter.
Before I started- the car ran just fine- strong acceleration- (hard pedal-to-
the-metal wind-out. NOT just mild acceleration..), crused fine, idled fine. I
would defy anyone to drive the car and acertain that the carbs needed any
adjustment at all.
However- I had made a new exhaust header and changed aircleaners... The books
say that these changes will affect mixture. Car ran strong. but I wanted to
KNOW that I was not going to burn a piston while racing or wash the rings (and
lose power) with an over-rich mixture. I wanted to KNOW it was right..
Now I had adjusted the carbs "according to the book". Set balance with air-
meter (Piece of hose, UniSyn or better- I use RedLine SyncroMeter). Then set
jets by the lift-the-piston method.
Now- acording to the books- the carbs were tuned. Don't believe it!
I installed the meter... & went for a drive.
The meter showed of-scale rich on WOT (reflecting the changes I had made in
the engine..) I sure would not have known this seat-of-the-pants..
I cranked the jet up until it showed around 12:1 on hard acceleration- now I
know I have the right mixture for max power on WOT (there WAS a big
difference- Cool!) and I am not going to hole a piston..
Most cool...
The car would not idle well now of course- cranking up the jets had made idle
too lean. It was also off-the-meter lean while cruising at 50 mph but I only
knew that from the meter. The car still drove fine and I could adjust the
idle screws up a bit and it idled OK- albeit a bit fast.
I am going racing so it would be OK as in racing you are usually on or off the
gas... Really hard driving was not going to hurt the engine as it was set. WOT
was "right" Still... I knew- but only from the meter- that mid-range was lean.
I looked at my needle selection & found one that was LEANER at the top end,
and about the same at mid-range- I zero'd the jets, installed the needles and
cranked the jets down an equal number of flats so it would run...
(I went from a #6 to EM)
Again took it on the road (I have a favorite uphill road too!). Made a couple
of runs of hard acceleration, adjusting the jets down until I was back at 12:1
on hard WOT.
Now it was also on-the-meter (around 14:1) at cruise. Cool... That's what I
was after.
The car was a LOT more responsive mid-range. I mean a bunch- & even my butt
could tell. Super-most cool... I would have never known what was there if the
meter hadn't told me.
Idle is a bit lean still- around 16:1- but I only know that from the meter...
It idles fine for me & it ain't gonna hurt the engine.
I can drive hard with no worry, the car is super responsive at mid-range &
starts & idles fine. That's what you are after- not a steady 14.7:1 reading.
In years past, I would have thought that the carbs were adjusted just fine at
the start. After I had "tuned" the carbs, just like the books say to do. Car
ran fine- plugs looked OK to me (limited plug-reading skills..) as they ran
rich at WOT & lean at mid-range and right at idle- hell, when I looked at 'em
they had run all over the scale & I guess it adveraged out cuz they looked
just like "the books" said they should. Yes, a "plug-cut" run would have shown
(an expert) that WOT was rich however, I hang with some hard-core sporty-car
types and have yet to know one who went to the trouble- ever!
"Tuning" SU's according to the book- only insures that your car will idle!
It really is reassuring to KNOW that the mixture is right. It is a fun pursuit
for Motorheads and very rewarding to the mind (and the car).
Fred Sisson
|