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Mark Taxis
Prolific User
Username: mark_taxis

Post Number: 138
Registered: 4-2007
Posted on Thursday, 04 May, 2017 - 11:08 am:   Edit PostDelete PostView Post/Check IP

I have been adjusting the mixture on my Mk 6 special, originally it was running too rich, I have leaned the mixture a bit, and after a long run I took out the plugs, see attached photo, the plug is a white / grey colour, is this, about, right? I have heard that these cars like to run a bit rich but I am not sure if this is true.
I use 91 octane fuel and a lead replacement additive to protect the valves


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Patrick Ryan
Grand Master
Username: patrick_r

Post Number: 1511
Registered: 4-2016
Posted on Thursday, 04 May, 2017 - 11:30 am:   Edit PostDelete PostView Post/Check IP

Hi mark,

The only thing you need to remember is that the leaner the mixture the higher the combustion temperature.
The electrodes look as though they have gotten pretty hot.

Have you seen bike or car headers that are running a carby for each pot.
You get excessive heat, and blueing of the exhaust pipe where that carby is to lean.
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Mark Taxis
Prolific User
Username: mark_taxis

Post Number: 139
Registered: 4-2007
Posted on Thursday, 04 May, 2017 - 12:27 pm:   Edit PostDelete PostView Post/Check IP

Patrick
I think I have made the mixture too lean, will richen it up a bit and see what they look like then
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Patrick Ryan
Grand Master
Username: patrick_r

Post Number: 1512
Registered: 4-2016
Posted on Thursday, 04 May, 2017 - 12:40 pm:   Edit PostDelete PostView Post/Check IP

Good idea Mark,

A slightly rich (not to rich of course) mixture keeps that combustion chamber temps down, thus lessens the risk of hot spots or piston grabbing.

I was trying to find a good image of exposed chrome headers on a typical 4 cylinder motor bike, that had one or 2 cylinders running to lean.
But couldn't find one
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Mark Taxis
Prolific User
Username: mark_taxis

Post Number: 141
Registered: 4-2007
Posted on Sunday, 07 May, 2017 - 10:14 pm:   Edit PostDelete PostView Post/Check IP

Retarded the ignition a touch and richened the mixture, nice light grey / browish colour now and car goes like rocket
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Patrick Ryan
Grand Master
Username: patrick_r

Post Number: 1524
Registered: 4-2016
Posted on Wednesday, 10 May, 2017 - 07:59 am:   Edit PostDelete PostView Post/Check IP

Good stuff Mark,
And you can sleep better knowing that its not as hot as hell in there.
Good job!

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