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Old 06-19-2002, 07:40 PM
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Colortuning Carburetors

Colortuning Carburetors
by Rick Jemison

("//" indicates future pictures or links)


  • The //ColorTune is a terrific tool for setting the basic idle mixture of any vehicle. In addition, it is effective for setting the off-idle transition mixture, and can even be used to check and adjust mixture up to about 4000 rpm.

Description of ColorTune Components:

  • The ColorTune is basically a //spark plug body with a quartz window. The unit has a //plastic body in //two pieces which helps in viewing the window which would otherwise require a darkened work area to view well. This plastic tube has a slot in both pieces for flexible spark plug wire //extension which gets the spark plug boot out of your way for easier viewing. The //top part of the two piece plastic tube has a highly polished aluminum plate attached on a hinge. This allows you to view the ColorTune window from any angle (very handy for those plugs that seem to be on the bottom of the engine).

  • The ColorTune also comes with a //cleaning kit consisting of some cleaner to clean up the quartz window.


  • To use the ColorTune, you simply remove a spark plug and insert the ColorTune plug. Tighten but not real tight (remember that window is part of the plug body and will crack easily). I like to warm the engine up prior to tuning. But, if you can't get the engine to run very well, pop the ColorTune plug in one of the plug holes and start from scratch.

  • Once the ColorTune plug is installed in the spark plug hole, attach the spark plug extension wire. This doesn't have to be real tight. Just snug it down finger tight. Slip the lower extension tube piece over the plug wire extension and seat against the ColorTune plug. Install the upper tube piece and align the wire slots so the plug wire extension can move in it. Move the tube so with the hinged plate adjusted properly and positioned so you can view the plug window comfortably. Now attach the plug wire to the extension. The //ColorTune is ready to use.

  • It is a good idea to make some initial adjustment of the //idle mixture screw prior to colortuning (so the engine will run). If you're not sure about this setting start my LIGHTLY (finger tight - and light finger tight at that) turning the idle mixture screw until it is seated. Then back the screw out 1 turn. If the engine doesn't run at this setting, increase by 1/4 turn until the engine will run.

The Tuning Process:

  • The tuning process begins with preadjustment of the idle mixture to some "best guess" setting (manual setting, if you've got a manual) or 1 turn backed out from a fully seated idle mixture screw.

  • The engine is then warmed up to allow the engine to idle without the choke.

  • The ColorTune is then installed, one cylinder at a time by replacing the spark plug in each cylinder with the ColorTune plug.

  • The idle is then set and the ColorTune color is read. Changes are made with the idle mixture screw.

  • When the color is adjusted correctly, the engine is turned off, the ColorTune is removed and replaced with the spark plug and wire and the ColorTune is moved to the next cylinder until all cylinders have been read.


For vehicles which do not have one carburetor per cylinder carburetion, the tuner must "split the difference" on the tuning; adjusting for the best "average" color that can be achieved with the carburetion/manifold setup on the engine. Perfect tuning may not easily be achieved with a single carburetor setup or a setup where carburetors are feeding cylinders indirectly through a plenum.

Idle Tuning:

  • Start the engine. Let the engine settle down. If it is on choke the ColorTune is going to tell you it is running very rich. Wait til the engine warms up and the choke is OFF.

  • Adjust the idle to the manufacturers recommended setting. If you're not sure of this, for cars try 700 rpm, for bikes try 900 - 1000 rpm.

  • Now, with the idle mixture screw initially set, the engine running warm and off the choke and the idle set at the correct setting, view the ColorTune for this cylinder. According to the ColorTune literature, the color you should be shooting for is a //cobalt blue. This may not necessarily be the best setting. But certainly is a good place to start.

  • The main thing is to be consistent. Shoot for the same color on each cylinder.


On the Yamaha XS1100s folks have discovered that cobalt blue settings don't run well on this bike. The engine on this machine likes a bit more fuel (a little richer). So tuners here go for just the emergence of a yellow //flame. Achieve this setting by turning the idle mixture screw. Left is richer / right is leaner (on every carburetor I've ever worked on!).

  • At this point, turn the engine off, take out the colortune. Remove:

    1. The plastic tube

    2. The plug wire extension

    3. The colortune plug

  • Then reinstall the spark plug (properly gapped and cleaned) and reattach the plug wire. Move to the next cylinder. Continue this process until all cylinders have been ColorTuned.


The following instructions should actually be performed during the tuning of each cylinder. The easiest way to accomplish this is to tune idle, then check the off-idle color (record this - write it down), throttle up to 2000 - read the ColorTune recording the reading, etc. As different carburetor circuits control different speed ranges, it may be easier to wait until all carbs are check and then a change made across carburetors. It is perfectly alright to tune a single carburetor until you're commpletely satisfied with mixture settings from idle to 4000 rpms if you'ld rather tune one carburetor at a time.

Off-Idle Tuning:

  • Off-idle is a transition area. The carburetor is controlling mixture partially with the idle circuits and partially with the mid-range circuits (some carburetors (4 barrel auto carbs in particular) had //off-idle circuits which enriched the mixture when the butterfly moved sufficiently to activate them.

  • These circuits are generally not easily tunable (with Weber being the obvious exception!)).

  • Therefore, a mixture change here is generally made with either

    1. a change in the mid-range mixture or

    2. a blend of changes in both the idle mixture and the mid-range mixture.

  • For this reason, off-idle adjustment is often held off until a check of mid-range mixture is made. Another consideration is the effect of snapping the throttle open from idle. Eliminating a hole here may require seriously enrichening an otherwise correct mixture setting.

Low Mid-Range Mixture Check:

  • I'll define this area to be from about 1800-2500 rpms. Here the mid-range circuits are taking over responsibility for mixture control although idle mixture is still a factor.

  • This area is therefore generally controlled by either a power valve or a needle arrangement (typical in most motorcycle carburetors). Emulsion tube tuning can take place here. This area is very important in quick throttle snaps from idle mixture quality.


  • Here idle circuits are no longer a serious factor in mixture quality. The //power valve or //needle valves are now in full control of mixture quality.

  • For the purposes of the ColorTune use, this area is from 2000-4000rpm. Generally, a reading at 2000 and another at 3500 - //4000 are sufficient to give a clear indication of the mixture quality of the mid-range circuits.

  • Changing the //needle position can be sufficient for most tuning unless major engine modifications have occurred. This should never be taken as gospel however. In order impact to mixture quality at this range it may be required to change the needle; change and/or modify the emulsion tubes; or change the main jet.


The ColorTune provides an easy quick method of checking mixture quality in the idle, off-idle, low and mid-range engine speed ranges (idle to about 4000 rpm). The ColorTune is not really useful above about 4000 rpm as it is not easy to see changes in mixture through the window at these engine speeds.

Good Tuning!


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