what are my old spark plugs telling me?

maxgreenwood · Post by **maxgreenwood** » Wed Jun 01, 2011 10:35 am

Sorry for slightly blurred pic. They came out in this order looking at engine from front of car. Left and right-most were quite white - weak mixture? Middle two are quite carbon-y. They should all at least show the same signs i think?

I don't have any experience with petrol engines, but i would think the carb / distributor is not providing the same mix to each cylinder? It does run a little rough on a steady light throttle, ok when more fuel pumped in. And sometimes loses go for an instant like one cyliner is not sparking or something, but that's a guess as i say with my limited experience.

I have been advised to clean the rotor arm and should i possibly look at a new coil and/or ignition module?

Old spark plugs May 2011 by Maxwell Greenwood, on Flickr

Linegeist · Post by **Linegeist** » Wed Jun 01, 2011 10:46 am

Mine usually look the same.

I'd say that's fairly normal with a mass-produced inlet manifold built to a price. The two middle cylinders get the best fuel/air charge (the carb's right above them, while the two end cylinders have to make do with whatever they get - and after fuel droplets have fallen out of suspension due to tight bends (the Dyson effect :wink) cold and gravity.

A really weak mixture will show serious signs of plug overheating with bubbling on the ceramic insulator and VERY white deposits.

If the car doesn't stumble or hesitate in use, I'd opine that you don't have a problem there. The plugs are nice and clean too! If you're passing this way I'd be happy to hook my exhaust gas analyser up to your pipe (ooer Nurse!) but I think you'll find it's within tolerances.

:

Post by **Tim Leech** » Wed Jun 01, 2011 10:53 am

They dont look to bad at all really Max, I wouldnt worry. Its always a good idea to fit new plugs and maybe rotor arm and cap if you are unsure when they were last changed.

rayfenwick · Post by **rayfenwick** » Wed Jun 01, 2011 11:43 am

kiwi · Post by **kiwi** » Wed Jun 01, 2011 11:46 am

rayfenwick wrote:

+1

Linegeist · Post by **Linegeist** » Wed Jun 01, 2011 12:23 pm

Bandwaggoners!

Gibbo2286 · Post by **Gibbo2286** » Wed Jun 01, 2011 12:30 pm

What those plugs demonstrate to me is not so much from the electrodes but the outside of the insulators, the discolouration suggests 'flashover' i.e. the spark when the motor's under load running down the outside of the plug, it may be more evident in wet weather. Gibbo.

citronut · Post by **citronut** » Wed Jun 01, 2011 7:53 pm

as Gibbo says or even valve clearance's,

rotor arm wont give cause plugs to varry in colour between each other,

i would also do a compresion test,

regards malcolm

Linegeist · Post by **Linegeist** » Wed Jun 01, 2011 8:14 pm

If it was flashover/tracking, would there not be carbon deposition on the ceramic further up near the HT terminal? My experience of tracking is that you get fine black lines where spark erosion has pitted the insulator and deposited carbon - which leads to even more tracking. This is especially evident inside distributor caps where moisture has provided the initial earth leakage path, and carbon deposition has made it permanent - you can see the fine, black, spider-webbed tracks and, often, scraping them away will restore the cap's resistive integrity long enough to get you home.

The discolouration where the plug caps end is, I suggest, nothing more than the oil mist found in most engine compartments baked onto the hot insulators.

citronut · Post by **citronut** » Thu Jun 02, 2011 8:02 am

yes Bob but if your porcelain has dirt/oil deposits around the kneck it can lead to/cause tracking, even more so if it becomes damp,

regards malcolm

JayW · Post by **JayW** » Thu Jun 02, 2011 8:10 am

I'd say from the age / type of motor thay look perfectly NORMAL and to suggest anything dramatically otherwise would be scaremongering.

No-one even bothered to ask the most obvious question of all... How many miles they've done...

Linegeist · Post by **Linegeist** » Thu Jun 02, 2011 12:33 pm

citronut wrote:........yes Bob but if your porcelain has dirt/oil deposits around the kneck it can lead to/cause tracking, even more so if it becomes damp ..........

I agree entirely Malcolm, but that's not what Mister Gibster said.

the discolouration suggests 'flashover' i.e. the spark when the motor's under load running down the outside of the plug

I agree that those deposits may LEAD to flashover under wet conditions, but they aren't CAUSED by it! HT Arcing leaves distinctive track marks, not bands of simple grime like we see here.

Same horse, different nosebag?

Post by **Tim Leech** » Thu Jun 02, 2011 12:44 pm

Calm down dear

Max, just buy some new ones mate!

maxgreenwood · Post by **maxgreenwood** » Thu Jun 02, 2011 2:40 pm

I have! - and fitted. Very little dicernable difference in running, still got the hesitation / not so smooth running on a steady throttle, i would think the jets need cleaning out? Would some of that cleaner additive like Redex do anything worthwhile? Maybe at a higher concentration than recommended on the bottle? I have some.

Post by **Tim Leech** » Thu Jun 02, 2011 3:18 pm

I would undo the airfilter pipe from the top of the carb, one hex screw and a jublilee clip, pull it off and spray carb cleaner directly in with the car running, dont over do it though or it may stall, ideall I would keep the revs up whilst you do it.

Is the hesitation there all the time or just when its warm, could be that someones fiddled with the mixture and made it run too lean, my 16TRS was like that, it flew through the MOT but they did say it wa running on air!

so I tweaked it up a bit as it was rather flat.