Tune Your Engine Using a Colortune Spark Plug

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Have you ever thought to yourself....self, it would sure be nice to see inside the combustion chamber when my engine is running. What better way to adjust the mixture! Well, think and ye shall receive. The "Gunnison Colortune" is a tool that's been around for a very long time, used mostly by sports car owners with multiple carburetors to balance them. Now Gunnison has heard the thoughts of motorcyclists and has produced a product specifically for us.

First a heads up, this little gizmo is pricey (about $65) if you only use it once, set your mixture and put it on a shelf. You'd be far better off (financially) if you can get some riding buddies to go in on the order with you then everyone can share. I got mine from Auto Expert Products (site no longer online) but shop a little in case there is better pricing elsewhere. Here's one from Amazon.

What you'll need

  • 1- Gunnison Motorcycle Colortune ( 12 mm for Vulcan's )
  • 1- 16 mm Deep Socket
  • 1- Long Nose Pliers
  • 1- Carbed Motorcycle or Aftermarket FI Computer (TFI, Cobra, Power Commander)


What It Is

The entire theory of this device is that the flame produced by ignition of gasoline in your cylinder will be a different color based on whether the engine is running rich or lean. Allow me a brief analogy here. If you are using Microsoft Internet Explorer right now, look at the blue bar at the top of the browser. Notice it is a very deep blue at the left fading to a lighter blue at the right?

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The lighter color as viewed through the clear Colortune spark plug would indicate a lean mixture while the deeper blue would indicate a perfect mixture. Beyond the deeper blue as the mixture gets richer (more fuel, less air) the flame begins to show yellow with an extremely rich mixture looking bright yellow as shown on the Colortune Instruction sheet at left.

The Colortune kit includes the spark plug (smaller for motorcycle applications so it

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doesn't affect combustion chamber size) a spark plug wire extension so you don't have your stock wiring in the way and a handy dandy viewing 'tunnel' with swiveling mirror on top for situations where you can't look directly down into the spark plug hole while fiddling with your aftermarket FI computer or the carb. There is also a spark plug socket adapter to convert the plugs 16 mm hex into a standard 13/16 spark plug socket size. You can't use this on a Vulcan because a 13/16 spark plug socket will not fit in the hole.


How To Do It

  1. Warm your bike to normal operating temperature (waiting until the fan comes on would be an indicator you're warmed up if you can't just go out and put ten miles on the bike). Remove one of your spark plugs. For most carbed applications you'll find the right side rear is handy because (on a Vulcan) the plug hole isn't nearly as deep. If you have an FI computer which cylinder to use will depend where you mounted the computer. If it's on the left side of the bike then the left front plug would probably work better. This could be where the viewing extension and mirror will be handy too.
  2. Use a little dab of grease or Vaseline to hold the gasket on the Colortune spark plug. If you don't you could have trouble getting the plug lined up with the gasket and then with the hole.
  3. Using a standard 16 mm deep socket, screw the Colortune spark plug into the hole your stock plug came from. Do >NOT use a wrench to tighten the Colortune. Just snug the plug down using hand pressure only. Turning the Colortune plug with a wrench can break it.
  4. Attach the spark plug wire extension to the Colortune plug (it screws onto the top) and attach your stock plug wire to the other end. Try to keep your stock wire away from air cleaners and other metal parts to avoid the possibility of the spark arcing to ground. A spark could mar an expensive chrome part. Also while conducting the test avoid touching the extension with your hand or other body part. You will get knocked on your butt by the shock and you'll tingle for an hour, trust me here ok?
  5. Gentlemen and ladies, start your engines!

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Take a look inside the spark plug hole and you should see something like this. Colortune calls that "Bunsen Burner Blue". A good many of us have never seen a Bunsen Burner but it doesn't matter too much, you'll know it when you see it.

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

You're going to start the tuning process by screwing up your mixture,

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making it rich so you see a yellow flame like the photo at right (sorry, I took pictures of the box so they aren't all that great but hopefully you get the idea.) Now just lean the mixture out until all the yellow flecks are gone. You'll have a deep blue ignition and a perfect mixture. I found running the idle up to about 1800 rpm worked best with my 1500 but you can experiment.

Here are AVI videos (you'll need Apple Quicktime V7.0 or later to view) of the Colortune in operation using my Nomad.

This Video begins with all pots on a TFI turned off with the engine at fast (about 1200 rpm) idle. The #1 (green) pot is turned clockwise slowly to the 4:00 position where you'll see the flame has gotten very yellow (rich). The pot is dialed back again with a stop at "2" (barely any enrichment) and finally back to "1". My final setting after doing this under load (and confirming the setting on a dyno is 2:15 for #1)

This Video shows the roll on and higher speed mixture. The #2 (accelerator pump) pot is off as is the #3 (high speed) pot. Notice as the throttle is twisted quickly a couple of times the flame turns very yellow (rich) so it appears no enrichment is necessary under acceleration at least with no load. As the throttle is turned up slowly revs are taken to 4500 rpm, well into the #3 pots zone had it been turned on but again, no enrichment appears necessary as the flame is still on the edge of being slightly rich. My final setting after doing this under load and confirmed on a dyno is 2:45 for the #2 pot).

Once you have your idle mixture set you'll want to run the rpm up to where your main jet kicks in. If you have an aftermarket FI computer you'll have set that number yourself. On the TFI a red light will come on when the 'main' jet is working. Check the flame again and go through the same process turning the appropriate mixture pot of your computer to make the engine rich then leaning it out. You might discover (as I did) that the engine is plenty rich at higher speeds and you can turn the enrichment entirely off. If you have a carb it's a little more complicated because you'll have to rejet between tests. My #3 pot is set at 1:00 (off) #4 is at 9:00 (4000 rpm) which keeps the idle enrichment in play to that point.

Setting your 'accelerator pump' takes a little fiddling. The idea is to see a yellow flame (rich) when you crank on the throttle but then see the flame turn blue as you reach a steady speed or let off the throttle. I ended up turning my enrichment (TFI) down a lot from where I'd been running it after doing this test.


Poor Man's Dyno

Ideally your bike would be on a dynamometer when you're conducting these tests (except idle) so there is a load on the engine. If there's one in your neighborhood and the guy who owns it will let you make a few runs for a case of beer then by all means, go for it. There are alternatives albeit possibly dangerous ones so for the record, I didn't tell you to do this.

If you jack up the rear wheel of your bike with the front tire against something very solid like...oh, say, the washing machine then, (in theory mind you), you could sit on the bike and, engine running, let out the clutch and shift up to your top gear. And then (theoretically speaking) you could give the bike some gas running it up to a 'cruise speed' then apply some rear brake to simulate road friction and wind resistance. Obviously you wouldn't want to do this for long unless you're planning to replace the brake pads anyway. A friend can check your Colortune plug to make sure you still have a nice deep blue ignition happening in this 'cruise' mode. Doing this you'll discover that load does play a part in the engines needs and you'll end up setting your TFI (or choice of aftermarket fuel enhancement device) richer than with no load.

If (you daredevil you) you've chosen to try the poor mans dyno and all was well then you're finished! If not you probably know whether you're lean or rich so make your adjustments.


Finishing Up

You may be wondering why you needed the long nosed pliers in the 'what you need' list. If so then you haven't removed the Colortune plug from the cylinder yet. The washer has a tendency to stay down in the plug hole. You'll need those pliers or a piece of utility wire to fish it out.

Caution, the Colortune plug is probably going to be hot. It will also be a little loose in your standard 16 mm socket so have a place to set both the socket and plug until things cool off. Dropping the plug will be a major fubar as it can crack the clear crystal window. Placing the hot plug back in its soft plastic case will deform the case.

Put some anti seize on your stock spark plug and put it back in the hole, torque it to 13 inch pounds. Reconnect your plug wire and go ride knowing you have a perfect air fuel mixture.

Offer the use of your Colortune to your friends in exchange for a bottle or two of your favorite beverage especially if they're doing a Valkeryie with six cylinders and six carbs.


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