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Spark plug in cylinder 6 is black... why ?


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Hello,

 

I'm not really having any issues with my TR6 other than the fact that the spark plug in cylinder 6 is rather black (dry). And this has been so for a couple of years.

 

The engine has been fully rebuild and is fitted with 2 SU HS6 carbs (bought them new a couple of years ago) and 123-electronic ignition dizzy. Other than that it's a standard engine (US spec)

 

Spark plug 1 through 4 are perfect (light brown). Number 5 is a bit darker an number 6 is quite black but dry. Plugs are NGK BP6ES.

 

What could be the cause of this ?

 

The car starts and runs very nice, doesn't burn oil and does about 24 MPG.

 

Cheers,

Edwin

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Edwin

 

Check the throttle bodies are synchronized and letting same volume of air into 5 and 6

 

If the spindles or linkages are worn and the butterfly plates arent opening with the others the lack of air causes the over fueling mentioned by Alec

 

There was a thread on here recently with an air flow meter recommended - I'll see if I can find it and add to this.

 

Here it is. See post no 2 has a link to te meter. I bought one. Works a treat!

 

http://www.tr-register.co.uk/forums/index.php?/topic/52163-inlet-manifold-balancing/

 

Graze

Edited by Graze
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Indeed... it's on carbs and have checked the air flow on both carbs. They are perfectly synchronized both at idle and at about 2000 rpm.

 

And it must be that the mixture in 6 is too rich. But i'm a bit puzzled as to how only number 6 is getting less air or too much fuel.

 

If the setup of the rear carb would be different from the front carb shouldn't plugs 4, 5 and 6 show the same issue ?

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Plug lead or plug or compression problem.

Stuart.

+1

and check the disy cap isnt 'tracking' HT to earth on #6.

http://s789.photobucket.com/user/Jaysgarage77/media/Badgersport/madness3007.jpg.html

note fine lines of carbon on old cap on right of page.

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Some say - and I'm not talking Stig - that the plenum starves No.6 of air, the previous five robbing all that can get in.

Try a run without the plenum, and another plugcheck.

John

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Some say - and I'm not talking Stig - that the plenum starves No.6 of air, the previous five robbing all that can get in.

Try a run without the plenum, and another plugcheck.

John

Keep up John its a carb car.

Stuart.

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Just checked compression and readings are exactly the same on all cylinders and I know it isn't a faulty plug because I changed the plugs a while back ago and no difference.

 

But I also tested the plug leads with a multi meter and voila... plug lead on nr 6 is faulty. The connection of the wire at the plug end is loose so it sometimes makes contact and sometimes doesn't....

 

So I'll be ordering a new set of leads.

 

Thanks !!!!

 

Cheers,

Edwin

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