Author Topic: Using colortune to jet a carb.....article inside  (Read 1768 times)


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Using colortune to jet a carb.....article inside
« on: December 04, 2004, 11:37:36 AM »
Interesting  :)

Article I found online

Tuning power/economy with a limited budget
        Mixing it on your Own Dyno
        by David Vizard
        The average home tuner's most frequent problem, if the letter
        we get are anything to go by, is getting the fuel/air ratios
        (mixture) right after having tinkered wit hthe motor.  After
        printing this article, we do not expect to get, ever again, a
        Technical Query asking what needles or jets to use on a
        modified motor, becuase this really should be the end of your
        mixture problems!

        When tuning the engine, it is more than likely that the
        fuel/air demand of the engine is changed.  Becuase of this it
        is neccessary to "calibrate" a carburetter so that it passes
        fuel into the engine in the correct quantities for the amount
        of air being consumed.  Over its operating range, a
        carburetter may have to pass a volume from say two to two
        hundred cubic feet of air a minute!  At tickover, the
        consumption of air is very low, but at full throttle the
        opposite applies.

        At every point between the two extremes the air flow
        requirement is different, yet at all these points the
        carburetter is expected to mix the correct amount of fuel with
        the incoming air.  There is little point in trying to set up
        the mixture at tickover becuase very few, if any, of us drive
        on tickover and there is absolutely no guarantee that the
        mixture is right throughout the range.

        How do we know or how can we tell what the mixture is like
        anyway?  Up until now, the most common method has been to
        "read the plugs".  If you have had years of practice and are
        an expert plug reader (not the print on it) you can just about
        pull off the job of mixture setting.  The trouble, however,
        with a plug count is that one is never too sure what the plug
        is saying.

        For instance, a dark, very slightly sooty plug can look like a
        mixture that is just a shade rich, but it could be that the
        mixture is just right and the plug type is too hard (runs too
        cold) for the motor.  Unless you are an expert then, reading
        the plug is, to say the least, just shade dodgy.

        An alternative to reading a plug is to get your engine set up
        on a dynomometer; either the rolling road type or the engine
        type.  When an engine is on a dyno, it can be run at various
        rpm and against various loads, thus simulating the conditions
        met on the road.  While all this is going on, an electronic
        gas sampler can be analysing the exhaust and indicating the
        fuel/air ratio.

        Going from reading plugs at a dollar a time to using three
        grand worth of dyno might seem like going from one extreme to
        the other or to use a comparison, going from an abacus to an
        electronic computer.  What is needed to use the analogy again
        is a slide fule ie: a method between the abacus and computer,
        or to come back to reality, a method of setting the mixture
        which is more akin to the dyno and mixture analyser than the
        plug reading method.

        A dynomometer measures horse power, commonly called brake
        horse power becuase a dyno is nothing more than a brake,
        calibrated to read out the work absorbed and the rate of
        absorbtion.  Here we have a clue; all cars have brakes (or
        should have) so what we have, in effect, is an uncalibrated
        dyno fitted to our car.  To simulate road conditions whilst
        standing still, all we need do is to jack up the driving
        sheels of the car, put it in gear and use the throttle as if
        we were driving along the road.

        To simulate road levels we need only apply varying pressure on
        the brake pedal.  Such action will, of course, get the brakes
        hot.  For our purposes we need to be able to hold full power
        for about fifteen seconds maximum.  If your brakes cannot cope
        with this from the heat point of view, then you are sadly
        lacking in that department so see to it.  It's probably better
        brakes you need and not more power!

        Okay, so we have our dyno.  The fact that it does not read out
        in horsepower is, for our purposes, irrelevant.  What we need
        now is a mixture analyser.  Up until a few years back this
        would have cost a tidy sum.  These days we have a device known
        as a "Colortune" and within the price range that can be
        afforded by the enthusiast, this is the _only_ device we know
        of that will do the job in hand.

        For those who man not know, a Colortune is a device which
        replaces the sparkplug in the cylinder.  The top of the
        Colortune is made of a Borosilicate glass and this allows you
        to see what is going on in the mixture combustion chamber.
        Different mixture strengths burn at different colours.

        By looking into the combustion chamber through the Colortune
        we can get a good indication of the mixture strength
        prevailing.  Inspection of the flame color shows that four
        fairly distinct stages occur.  when the flame colour is
        blue/white, the mixture ration is between 16 and 14:1; a blue
        colour indicates a mixture strength between 14 and 12,5:1; a
        blue/orange colour indicates 12,5 to 11:1 and orange indicates
        11:1 or less.

        The Colortune, then, is indicating at the colour transition
        points the mixture strength of the ingoing charge.  Maximum
        power occurs when the fuel/air ratio is between about 12,5 and
        13,5:1.  The exact point varies from engine to engine, but
        most cast iron tuned production engines seem to be best around
        12,8:1.  The best economy is achieved on weaker mixtures than
        that giving maximum power, and fuel/air ratios between 14 and
        16:1 seem to be the easiest on the pocket.

        To set up the mixture in the manner about to be described you
        will need an accomplice.  The first and essential step is to
        part the car in some place which is poorly lit, so that you
        can see the combustion colours.  Jack up the driving wheels of
        the car until they are just clear of the ground and in the
        interest of safety, securly blockthe car so that it cannot
        move under any circumstances.

        At this point, warm up the engine, the remove a spark plug and
        replace it with a Colortune.  Set up the mixture so that you
        have a fuel/air ratio of about 12 to 12,5:1 (orange/blue) at
        normal tickover revs.  If you have multiple carbs you will
        have to do this for each cylinder or set of cylinders having a

        Next, get your accomplice to put the car into gear, usually
        third gear is best, and increase the throttle opening but at
        the same time put on the brake. (Continue opening the
        throttle and increasing breaking pressure until your
        accomplice ends up with the throttle wide open and the revs
        pulled down by braking to 2000 rpm).  You can now look ath the
        Colortune and at this point it will reveal what the fuel/air
        ratio is under the prevailing conditions.

        It can then be noted, preferebly by colour rather than
        reference to its fuel/air ratio.  After this, let the brakes
        cool for a few minutes, then repeat the procedure at 3000 rpm
        then at 4000 rpm and finally at 5000 rpm, stopping to let the
        brakes cool between each run.

        As far as brake overheating is concerned, it should not take
        more than fifteen seconds to ascertain the mixture ratio at
        each rpm interval, so they will be well within their capacity.

        Once you have an indication of the state of affairs of the
        fuel/air ratio up the rev range to 5000 (5000 rpm is the limit
        on the Colortune) the necessary corrections can be made to get
        it right.  Not only can the full throttle conditions be
        catered for in this manner, but so can part throttle and
        transient conditions.  For instance, a hesitant pickup when
        going from parth throttle to full throttle could indicate that
        the mixture is too weak during the transition from one state
        to the other.  During the transition period and for a short
        while after the Colortune should show a rich mixture
        condition.  If it doesn't, then you can bet your life that on
        a fixed jet type carb, the accelerator jets or pump stroke are

        On carbs like the SU and Stromberg CD variety, a lean mixture
        during the acceleration phase would indicate that the damping
        is insufficient.  A thicker oil is usually required to
        compensate this.

        A couple of cars were used as guinea pigs to test the method.
        Both cars were modified and therefore required different carb
        settings.  In each case the carburation has been originally
        set up by the owners who had only an average working
        knowledge of what was required.  After use of the Colortune by
        the mothod just described, both power and economy were better
        between 5-8bhp and 10-15 miles per gallon.

        The reason that consumption was so much better after use of
        the Colortune stemmed from the fact that the mixture was
        originally set rich for maximum power and unfortunately
        becuase of the guesswork method of setting, it was too rich.
        The performance increase was better than the power increase
        alone suggesting that the mixture was right (within limits)
        throughout the rev range.  As a side effect this led to a
        smoother running engine with a snappier throttle response.

        To sum up, the Colortune proves to be a very useful device.
        It can, at a price of 4-87,5 UKP easily justify its place in
        the tool kit of any self-respecting enthusiast, and its
        intelligent use can only bring about an increase in

        Editor's note:

        Whilst David Vizard has used this tuning method successfully
        with both a Mini and Austin 1100 its use on many other types
        of car could possibly be dangerous.  Firstly therefore we
        recommend that the suspension characteristics and drive
        shaft-prop shaft geometry of the car in question be checked
        carefully ebfore even considering use of this method.

        On rear wheel drive IRS cars which may have large wheel
        angularity, attempt to jack the car at points on the
        suspension that will allow the wheels to assume a position in
        angle similar to that which exists in normal use.

        On a non-IRS rear wheel drive car jack under the springs on
        either side at the axle location point.

        Make sure jacks are very secure and will not move under
        testing vibration, also make sure any car to be tested has
        alternate props underneath in case of jack failure -- ie:
        spare wheels and tyres which should be first tested using the
        full weight of the car.  One advantage of the DV method is of
        course that during load tesing the wheels on the ground have
        the brakes applied.

        On a Mini the best method Vizard found was to put a piece of
        wood on top of trolly jack lift point and jack up from the
        Mini sump.  Once having jacked car up, jam wheels and tyres
        underneath car for safety.  Car does rock but cannot thus fall
        over.  Drive shafts will assume peculiar angles during testing
        and this can be minimised by supporting bottom suspension arms
        on axle stands --these can however move and you may have to
        take the risk of the odd shaft angles for the few moments of

        This method does not apear to be practical with swing axle IRS
        cars ie: Herald, Spitfire, Vitesse Mk 1, GT6 Mk1.


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Using colortune to jet a carb.....article inside
« Reply #1 on: December 06, 2004, 04:54:03 PM »
I remember using these on cars I competed in about 20 years ago (I hate it when I say something that makes me feel old!).  Like most good ideas they're fairly simple.  If you've ever used a bunsen burner you'll understand (and picture) the different colour flames straight away and you'll readily know what a flame which gives of plenty of energy (heat in the case of a bunsen burner) looks like.  

I've often wondered  how useful they'd be in a two stroke.  My guess is that these days, with the advanced oils that we run at 40 and 50:1, they would be useful.  I don't know if any come with an indication of the colour flame you'd be looking for with oil (2 stroke) in the equation.

I know they work well in four strokes though!


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Using colortune to jet a carb.....article inside
« Reply #2 on: December 06, 2004, 06:03:37 PM »
They say they work good for 2 strokes as well and come with a flame color chart...very interesting I thought


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« Reply #3 on: December 29, 2004, 06:52:31 AM »
Many years ago I put a fuel injection system on my 1947 Harley Knuck.
It worked real well right up until the glass yellowed and then a litle while later it cracked. So if your going to use one remember they can only sustain the heat for a short period of time. BTW I took the F/I off and put a carb on it and it never ran better lol.