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colour of spark plugs


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Hi everyone


I have a Vitesse Mk1 2 Litre


My spark plugs are all a sooty (with bit of brown on the centre electrode) colour after town driving which I understand is not uncommon.


I did a couple of plug chops while cruising at around 3000rpm to try and get a more general picture.


Plug 1 - A very pale grey all over

Plug 2 - A darker grey

Plug 3 - Some where inbetween the first two

Plugs 4,5,6 not too bad a brown colour with some soot (all varied slightly, Is this normal due to different valve wear?).


I was a bit concerned about the first 3 plugs looking weak running and opened up the jet 1/4 of a turn


I did another plug chop and looked at no 1 plug (as this was the one that looked worse origanaly)


It was sooty all over, apart from the tip of the earth electrode, which was very pale grey


Is this running rich or weak?.


Is this something to be concerned about. I don't drive the car hard.


Also if it was running a bit rich on the second carb is this much of a problem (the car does the MPG it should).


I think the carbs may be difficult to set up in the usual way (if at all) as have wear in both throttle spindles at present.


As always any advice, very much appriciated.









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With worn spidles the idle mix is goingnto be a bit out, a simple way to improve this is to upmthe idle speed for some tests make it 1000 and adjjust the mix for best running .


to avoid soot , dont check anything with town driving or accelerating, the engine oil in the dashpot

gives a rich mix when opening the throttles.


use a 97ron or higher fuel and set the timing as near spec as possible without pinking



use a 95 lawnmower fuel and expect soot.due to slow burn and retarded timing


if you dont have a balancerundo the connecting link get the throttle plates to close fully, by unwinding the idle screws , turn them in to just touch the abutment and turn each equal amounts like 1.5 to 2 turns

thisnsets the plates in a mechanically balanced state, this is how they would have been delivered to the engine line in the factory


air flowed carb settings were not in the trade till the 80s


male sure filters are clean, and nothing blocks the front face ports


if it has temperature compensators CDSE spec make sure they are closed, atnormal temp.



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  • 1 month later...

no,  it just saying thats what its does and why its there,, its supposed too,  this is your accelerator pump equivalent on a down draft, 

thin oil wont damp the rising piston speed and you get a weak mixture when accelerating,  this would show up as a flat spot or less responsive 

under acceleration you need a much richer mix and the engine oil is there to slow the rise of the air piston, the extra airflow under its base drags more fuel from the jet 

same applies to Stromberg or SU 


the actual spec is a straight  sae20 but thats hard to find,   most use std 20/50 engine oil


anyone who recommends 3 in 1 has a problem elsewhere I wont hazzard a guess as to what the viscosity of this becomes when heated 


        cats pee comes to mind...  and no damping ability 


   if that makes any sense  



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