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MG TD TF 1500 - sparkplugcolour diagnosesc

i know there was a thread about the sparkplug diagnoses with a link to the colour diagnoses. I cant find the link in the archives...anyone knows and can tell me a link to sparkplug colour diagnoses?

Jan
jan mekes

Perhaps this one:

http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

Regards,
Dennis
D F Sexton

Dennis
I found it a very helpful guide but was unable to achieve 13-18 with unleaded fuel in an XPAG. After fitting Bosch Super 4 W56 plugs the engine seems much happier.
John
John James

This chart, as the others, was produced with leaded gasoline. The colour was given by lead salts which change the color depending on the temperature, and also which help to burn carbon.
The unleaded gasoline does not produced so much varying colours because the deposits are made of carbon only (also some deposits from the combustion of oil additives), and the carbon is black. You can only see if the mixture is too rich, if there is oil in the combustion chamber, but the colour of the plug does not vary markedly with the mixture strength to allow you to tune it accurately.
Jean-Marc Thély

Jean-Marc Any information on how lead additives affect the colors?
Bruce Cunha

Bruce
I was unable to obtain the "best" plugs conditions shown on the chart even with an over weak mixture, so I contacted Castrol and asked if Valvemaster might be partly the cause of the thin, shiny black deposit on the plug end rims and electrodes. They asked me to send one of the plugs for analasis and have had the NGK BP6ES for a couple of weeks now. When I receive their verdict I will post it here.
John

John James

Bruce,

The lead affects the colour of the plug as said. Different species of lead salts are formed according to the temperatures, the hotter being the whiter.

Lead, being a transition metal, acts as a catalyst for the oxidation of carbon, thus reducing the mass of deposit, hence the thickness of the layer on the spark plugs. That which allows you to see the colour of the ceramic.

Hence now without lead, apart extreme conditions, the spark plugs are not so talkative as in the past. On a wide range of mixture strength they are dark grey, and black if the mixture is really too rich. See the plugs of a modern car as an example.

Another fact of the influence of lead is the colour of the inside of the exhaust pipe. Lead has been withdrawn in Europe not so long ago, and we can remember that after a run on the motor way the inside of the exhaust pipe was light grey with lead content down to 0.4 g/L. At 0.15 g/L, it was dark grey. And now with no lead, it is black whatever is your driving style.

Anyway, taking out the plug is still the first thing to do check your carburetion, but to tune it precisely I prefer to use a digital tachometer to find the setting which gives the highest idle speed. I tried the transparent spark plug and was impressed how precisely they tell you that you are rich or not, but was unable to find the nice blue of the stoechiometric mixture.
Jean-Marc Thély

Sorry for the mistake. Lead is not a transition metal, but it does help to burn carbon deposits anyway.
JM Thély

This thread was discussed between 10/08/2004 and 16/08/2004

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.