Wednesday, July 20, 2016

Setting the Air/Fuel Mixture with Colortune

Following on from the previous post where I was faffing about with the ignition timing in the hope of getting my new engine running a little better, today I'll be turning my attentions to the air/fuel mixture and trying to get it optimised. To do this, I'll be using my Gunson Colortune kit I bought ages ago, so I'm pretty chuffed that it finally gets it outing.

Way back, earlier this year, I saw the Colortune kit hanging on the shelf while I was in Mini Sport and thought to myself: "Hmmmmm; looks interesting, I'll give that a go" so promptly purchased it on that very whim. Because of my lack of engine though, it sat on the shelf for months doing diddly squat, but now I have a working engine, it finally gets to have it's moment.

In the box
Opening the box reveals an interesting array of bits and bobs, the main parts being the see-through glass Colortune plug with copper washer and the extension H.T. lead. The other stuff I never really needed to use, but I suppose they might be useful with a different type of engine.

Reading the instructions, there is quite a lot of interesting information about the science of how the system works. Rather than rewrite the points, I'll just reiterate for your enjoyment as well as my own...
  • The correct proportion mixture is - 14.7 parts of air by weight to 1 part petrol (hydrocarbon fuel).
  • If there is perfect combustion, all the fuel will be burnt to produce carbon dioxide and water with no carbon monoxide or unburnt fuel (hydrocarbons)
  • The carbon in the fuel burns with oxygen in the air to produce carbon monoxide (CO), which then burns with more oxygen to produce carbon dioxide (CO2)
  • Hydrogen in the fuel burns with the oxygen in the air to produce water (H2O)
  • Nitrogen in the air passes through the exhaust with little reaction
  • When there is less air in the mixture, there is not enough oxygen to complete the burning process so some of the carbon monoxide is not changed to carbon dioxide and hyrdocarbons (unburnt fuel) may be present in the exhaust
  • Carbon particles glow yellow in the combustion of a rich mixture and in severe cases you may see black carbon smoke in the exhaust
  • When there is too much air in the mixture, it becomes more difficult to ignite, burns slower, and is therefore less efficient
  • The dilute mixture burns with a pale flame
  • Misfiring may occur and hydrocarbon levels in the exhaust gas will rise
  • Carbon monoxide levels stay low because there is plenty of oxygen available to convert it to carbon dioxide
So making sense of all that, we are looking for a nice bunsen blue flame. Orange and yellow are bad as is a light blue flame. I did start to read around the subject of combustion in greater depth, but quickly realised that understanding stoichiometry was not as straight forward as I'd first thought, requiring a decent understanding of both chemistry and the conservation of mass, neither of which are my strong points believe it or not.

So I'm happy to live with the simple knowledge that a Stoichiometric Air/Fuel Ratio is one where the correct amount of air and fuel are combined to produce a chemically complete combustion event.

Putting this into a simple table:

Applying these combustion colors to the graph supplied on the back of the Colortune box, we get something like this...

The practicalities
So there you have the science part, but what about the practical... Well I have to say that I found using the kit very straight forward once I knew what I was doing. There is one slight tip that is worth mentioning before we start and if you pardon the pun, it's regarding the sparkplug tip! If you have an old sparkplug, it's useful to unscrew the tip and attach it to the end of the extension H.T. lead and leaving it there permanently. This will avoid having to nab one from your existing set of plugs each and every time you use the kit.

In the instructions, it says that the engine needs to be up to operating temperature, so after a trip around the block and back I started by removing sparkplug number 1 and screwed in the Colortune plug with the copper washer. It stresses in the instructions the importance of not overtightening the colortune plug and suggests using little more than finger tightness. This seemed a little too loose for my liking so I gently nipped it up with a socket just to make sure it wouldn't leak.

Next the extension H.T. lead was attached... like so. 

Now at this point, you could choose to use both the upper and lower part the viewscope. I tried various combinations and found it easier and more practical just to use the lower part. Next the H.T. lead could be attached onto the tip of the extension.

Now it's time to start the engine again and see what's happening...

Looking down the view scope, all I could see were flashes of orange/yellow, which if you've been paying attention, is bad as it signifies that the mixture is too rich!! Well done if you remembered!

Orange - Too rich
Yellow - Way too rich!!!!

Correcting the mixture
Now to correct the mixture, I need to turn the aptly named 'mixture' nut underneath the carb. Turning it anti-clockwise (as viewed from above) makes the mixture weaker/leaner, so turning it clockwise makes the mixture...? anyone... you at the back?... Richer!!! That's correct!!

I've read in several places to turn it just one flat at a time and wait about 10 seconds for the corresponding color change to stabilise. As I weakened the mixture, it started to slowly turn from the orange to the required bunsen blue.

Bunsen blue - Just right

... and as I turned even more, it weakened the mixture out to the point that it was too lean and turned the flame more towards the light blue end.

Light Blue - Too Weak.

In order to get the mixture spot on, you need to find the mid point in the bunsen blue range. Unfortunately, that's a little easier said than done as the colors change so subtly when the nut is turned. I found it a bit tricky to find the 'edges' of the range; for example when the bunsen blue gives way to a lighter blue at the weak end or when the orange starts to come at rich end.

Now this is where some trial and error was needed. As I moved the mixture nut up and down, over and over, I started to get a feel of the bunsen range. Once I was satisfied with the colors I was seeing at either end of the range, I made a count of how many flats I had to turn in order to get from the weak end of the range to the rich end and halved the number to find the mid point. Once the nut was readjusted with the required turns, I was there.

Other Tests!!
As I was just getting acquainted with my Colortune kit, I chose to keep things nice and simple and performed the whole procedure with the engine at idle. Within the instructions, there are further tests that can be carried out at part throttle and full throttle, but for now, I'm happy with this simple tune up. Who knows, maybe I'll explore those other options later when I've got a bit more time.

So there you have it, another job ticked off the list and the car drives much better as a result. Now to get some miles on the new engine and get it properly run in.

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