On Wed, 10 May 1995, Will Zehring wrote:
> Fellow fiends:
>
> recent list quotes:
>
> >the MG idles best with an ORANGE flame.
>
> and...
>
> > The only time I
> >can think that you'd want a rich mixture is during acceleration. Sure, a
> >rich idle mixture will probably idle smoothly, but I worry about plugs
> >quickly fouling with carbon, not to mention wasted fuel.
>
> I was talking with members of my local car guy support group regarding the
> colortune and the rather high idle I have my car set at (approx. 1100 rpm).
> The general consensus here too is that the color tune is not really spot on
> for mixture settings on the B. It will bring you close, but probably no
> cigar. I trust to inspect spark plugs after reletively long bouts (30 miles
> of highway, for instance) of driving and "flat by flat" come around to the
> "right" setting. I'm at a nice dark brown now with champion N9Ys.
.......
>
> Will "Don't answer that!" Zehring
>
I'll jump into the fray regarding colortune and it's suitability for tuning
MGBs. I use colortune to put my car in "ROUGH" tune, when for some
reason I've had to mess with my carbs. ie. it's close and good enough for
the short term but really isn't bang on. For that really good tune I
take to a friend of mine who runs a local garage and specializes in
(gasp) V****s and British cars. He uses amoung other things an exhast
gas analyzer when setting up the car.
As with any other car there is also no point adjusting the carbs unless
the rest of sytem is set up and working perfectly. I.E. is the timing
right, do you have a good distributer good wires etc. and especially are
the carbs in good shape ie. no leaks around the throttle shafts, and jets
that aren't worn (A good bet they are if they havn't been replaced
recently) There is no point putting a whole lot of effort into tuning a
car until all these items are set up right.
Now on to tuning SU's. Many of you know far more about this than I do.
I'm certainly not an expert so correct me if I'm way off base.
This is for those that may be new to SUs. The mixture in an SU carb is
controlled by the location of a tapered needle in jet (a brass tube).
This location is changed by the amount of suction which casues the piston
holding the needle to raise or lower thus controling the mixture. Tuning
an SU generally consist of balancing the carbs, (i.e. ensuring that in
multiple carb set ups the initial amount of air flow (which is controled
by the butterfly valves which are controled by the gas pedal) is the same
for all carbs.) and setting the location of the jets to obtain a good
mixture (This is done by turning the jet adjusting nut).
Unfortunatly the mixture control accross the range of engine speed is
really dependant on having the right needle. These needles where chosen
years ago by the factory to represent the best air/fuel ratio for normal
operation. However very few cars are in their original factory shape.
Engine wear has occured, exhast systems have been replaced, the gas is
not the same etc. Thus the needle in the carbs may not be the right ones
for the job.
With my car for example the original needles where FXs but my car
due to a number of modifications (K&N filters, free flow exhaust,
ported and polished head and 20 thou oversized pistons) needed a richer
mixture accross the operating range I had to switch to a different
needle. Choosing a needle can be complicated I purchased a number (4 I
think) which had slightly richer profiles than the FX needle and my
friend and I tried them using the exhaust gas analyzer to see their
performance accross the operating range, and picked the one which had the
best match (He picked it not me). The result in driving the car was
noticable.
Just my 2 cents worth (that only 1.64 cents U.S. )
Gary Burrell
68 B
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