Lotus Elan

Checking Spark Plug Colour?

PostPost by: William2 » Sat Jun 24, 2017 11:24 am

What is the best way of checking mixture using plug colour as a guide? After a run to warm the engine up to normal temp for instance, if you left the engine idling for a short period would you expect the plug colour to be the correct brownish colour to show that the idle mixture is correct? The reason for asking is that I have used a Color Tune to get the correct Bunsen blue colour at idle but as far as I can tell the car is only doing about 22mpg when driven carefully below 3000rpm i.e. before the main jet circuit kicks in. I get the impression that there are mixed opinions as to the accuracy of the Color Tune method.
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PostPost by: billwill » Sat Jun 24, 2017 12:01 pm

As far as I am aware the colourtune should be used to set the colour to blue with flecks of yellow.


MPG is probably always low at idling speeds.
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PostPost by: Craven » Sat Jun 24, 2017 1:43 pm

Idle mixture adjustment is just that, low speed running mixture is set by the slow running jets via the progression holes.
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PostPost by: RichC » Sat Jun 24, 2017 4:04 pm

so much more to mpg than just spark plug colour. how's your timing?
if spot on i'd go for weakening mixture until you get dropp off of rpm and then crank it open again 1/4 turn... this is only if your carbs are in reasonable condition and already have equal airflow balance
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PostPost by: RogerFrench » Sat Jun 24, 2017 5:25 pm

If you really want to check using plug colour, then get out on the open road, run at full throttle for a minute or so, then switch off and dip the clutch. Brake to a halt, remove the plugs and look at them, or put in different plugs and go home to look at them.
Way back when, it was known as taking a "plug cut". Tells you about mixture, and also if your plugs are the right heat range.
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