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Different spark plug colours from the same carb


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Hi everone


Most of my car (Vitesse) use is around the city I live and therefore the plugs are always sooty.


Went for a 250 mile trip at the weekend and the engine was hesitating under load at lower revs.


pulled the 2 centre spark plugs from each carb and both were rather white.


So did quite few plug chops on the center plugs and mixture adjustments (girlfriend not impressed, as our weekend away) until until plugs were brown and car running a lot better.


Had a look at all the plugs before I got to the city.


Plugs for cylinders 1 and 3 from front carb looked like they were running weak (2 still good) and plugs for 4 and 6 on the rich side (5 still good).


Tappets are set.


Would this difference be due to different levels of wear in the different cylinders, therefore sucking in different amounts of fuel/air.


I guess I need to aim to adjust so none are running lean, even if this means some are running too rich, as this will cause less damage (as long as not a bore wash scenario).


Any help would be great.


Cheers Dave





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Hello Dave.


The first point is that you are running 6x cylinders via 2x carbs (Stroms, I presume) of which not all cylinders are going to get the same amount of fuel / air mixture during delivery. The cylinders that are furthest away from the carbs are usually the ones that suffer with regard to input; whereas those cylinders that are closest to the carbs or have a direct inlet feed will fair better. It's a common situation. 


Engines that are fed by a carb inlet per cylinder (Webers for example) tend to run equally balanced provided the carb is in tip top condition and adjusted correctly. So a perfect example of that would be a Vitesse engine running triple Webers - 1x inlet per cylinder.


Spark plugs are not there just to provide a spark and are quite complicated little chaps with heat range and length etc. Can I check you are running NGK's and not Champion plugs, of which the latter are plugs of yesterday. NGK's have a far better heat range tolerance and as such perform their functions better. 


Are your carbs air tight ?? By that I mean spindle wear and associated gaskets in addition to being correctly balanced and tuned ?? Are the butterflies opening at exactly the same time and is the idle screw set correctly for both carbs to be in sync ??


Your air filtration (air filters) plays an important part for correct fuel / air delivery. one filter dirtier than the other impedes air flow so that carb runs rich. 


Carbs do not need to tuned rich or lean to perform properly; they need to be tuned correctly & balanced.


I doubt very much if cylinder wear will have a big an impact as you mention - that tends to be performance related and associated issues. you can eradicate that line of suspicion by a simple compression test - worth doing at any rate; at least you will now how each cylinder is behaving.


I'm sure you will get more info on this via other users, but this is a "starter for ten" and covers some fundamental points. 


You mention hesitating under load at lower revs, that could be fuel or more likely ignition related. Are running electronic or OE system ??





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Surely if the front 123 is on the lean side and the rear 456 is rich then the mixture on 123 want richening and 456 wants leaning


engine oil in the dashpots


With the filters on use the piston lifting pin, raise till you just touchy feely the piston and then raise the piston 1mm , never yank it up,


if the idle faulters 50rpm its lean if it rises 50rpm its rich


if no change its about right


make sure the plugs dont have an R in the suffix



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yes but there's seems disparity between the front set are overall leaner than the back set, the common link to the 2 and 5 is the balance pipe in the manifold

is trying to even out the base running which may explain why 2 plugs seem better than the other 4 if thats a clue





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Yes diaphragms , give then a stretch to see if any holes appear.


dave the balance tube is the raised alloy across the top of the casting yes the tube section with the

take off you mention it joins both banks of carbs, to aid any mismatch in , lets say balancing

the pair


do you have asmiths valve in the breather, is the diaphragm ok




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