The outside of a motorbike engine seen up close near the exhaust

Home Repair And Restoration

Spark Plug Colour

Job Date 31st March 2016

By Ren Withnell

Any good mechanic ought to be able to look at a spark plug and diagnose how well the engine is running. Apparently. Back in the day when I were a young trainee mechanic I learnt that if a spark plug came out of the engine with a light tan colour the air fuel mixture was about right. Too dark or black meant the mixture was too rich, and too light meant it was too lean. Back then the Haynes manuals had a page showing many spark plugs of many colours and conditions with a note showing what caused that particular state. Back then...back before the internet was even a word let alone a thing. We did have colour TV though.

I've put a new plug in the 125, judging by the state of the old one it was long overdue. I am slightly concerned about the colour of the plug I have removed. As stated above I have always been told to look for a light tan colour where this appears to be almost white. 

the tip of a spark plug from the CBF, in fair condition but the base is a bit whiteLean...or acceptable?

This indicates to me that the air/fuel ratio is lean. However let me emphasise the word *indicates*. Before I go mental trying to richen up the fuel mixture on a digitally controlled fuel injected 125 there are other reasons for this. 

Firstly the plug was removed after a motorway run. A 125 by it's very nature is pushing hard at motorway speeds and alongside the warmer weather this causes the engine to be hot. There is such a thing as a "cooler" spark plug which is engineered for better heat dispersal to cope with hot engine situations. The downside of such a plug is it runs too cool when the weather is cold and the bike is not under much load. Everything is a compromise it seems. Ideally the plug should be viewed after the most common riding conditions and in the UK that's cold, wet, miserable and in slow heavy traffic. 

Secondly my CBF 125 is a lean burning engine. If an engine is designed to return 140mpg then I suppose it's designed to run a tad lean. Despite the abundance of CBF 125's worldwide I'm struggling to find other people's spark plug colours as a comparison. What about my previous spark plugs? Well in 53,000 miles this is only the third plug I've fitted and I can't specifically remember being concerned about the other ones.

The tip of the spark plug showing some greyness you could call that "grey" and grey is almost as good as light tan.

There are surely other factors at play too that I have not covered, if any come to mind please feel free to add them in the comments below. What has surprised me is just how clean the spark plug is. After about 15,000 miles the gap has opened a little due to wear but there are barely no deposits on the electrodes or the insulator. I expect modern fuels with modern injection systems make for a much cleaner burn.

Reader's Comments

Trazymach84 said :-
Its normal for EFI bikes.
1/6/2016 8:45:59 AM UTC
Ren - The Ed said :-
Ya think Trazymach84? I'm glad to hear that, all this electronic super efficient stuff is still rather new to me. Good old do I sound. Thanks :)
1/6/2016 9:09:54 AM UTC
Doug said :-
Aye, I can second that - the same discussion was had on a Royal Enfield Bullet forum I frequent; EFI models have a grey spark plug colour, not the nice tan of carbed engines.
2/6/2016 7:35:55 AM UTC
Ren - The Ed said :-
Thank you Doug and Trazymach84. Yes I am obviously out of touch with current spark plug colour trends. There is hope that the CBF 125 will continue for a little while longer.
2/6/2016 9:37:20 AM UTC
Doug said :-
Just to add, I've gone to the light side and bought a fuel injected YBR125 to commute on (117mpg so far with my upright, lardy 18st on it - woo! :-) and the spark plug is the same grey you have.
26/6/2016 9:57:25 PM UTC
Ren - The Ed said :-
Thanks Doug. Now you be gentle to that poor YBR and treat it right.
26/6/2016 10:29:20 PM UTC
Doug said :-
I intend to, I've just bought it some crash bars and a fresh bottle of ACF50 for it to share with the Enfield. The good lady wife takes her CBT in a few weeks, and if we actually get a summer this year some country lane pottering will be had :-)
27/6/2016 8:55:47 PM UTC
Ren - The Ed said :-
That sounds idyllic Doug. Be careful though, if the good lady wife gets the bug she might prove to be faster than you. It is rather embarrassing having to ask the Mrs to wait for you at the next junction...
28/6/2016 6:11:09 AM UTC
Ted said :-
Oh for the days when we were even ALLOWED to tinker with mixture and idle speeds.
I remember riding an old motor scooter home with a broken throttle cable, I had to pull it out from the handle bar end, remove the side panel, lash the cable through the right hand side of the front of the leg shield, and operate the speed by pushing my right leg out sideways.....carefully ensuring that if I needed to stop and put a foot down it had to be my left one.

4 strokes n plugs have never been much of a problem, over heating a thrashed 2 stroke on a longer run would often result in a melted plug tip and at 2 shillings and 6 pence (12-1/2 pence in new money) spark plugs didn't get replaced as often as the should have.

One thing I did learn very quickly, not all NEW plugs work.
No good just putting it in the bike tool box and waiting to find out one cold dark wet night.

Whenever I was going to change a plug, I would start using the new one the day I bought it, (just to make sure) and the old one was put in the tool box, because I knew that was working and would probably get me out of trouble.

Even with my youthful exuberance I was told I must have sensitive hearing as at speed I could almost hear the fuel gushing through the carb.

25/10/2017 8:36:20 PM UTC
Ren - The Ed said :-
You know Ted - in some ways it is easier change fuelling today than it once was. If you get the kit - ie computer and/or power commander - and learn a little tech then you're away.

No carbs off change jet carbs on again. Just stop, plug in the computer, change some parameters on a spreadsheet and off you go. It takes bloody ages to wrestle a bank of four carbs out from under a tank and between the motor and airbox, it takes a minute if not less to connect the laptop.

As for the notion of carrying a a spare plug. That just makes me hate 2 strokes even more.
26/10/2017 6:31:00 AM UTC
tahrey said :-
I wonder even if the general formulation of "modern" fuel - that is, lead free and high octane - makes a difference? I can't remember what colour exactly the plug was last time I checked it on my CG (carbed of course) but I'm ~fairly~ certain it was more grey than beige. I don't know if it's the case for Tetraethyl Lead, but I know common Lead Oxide is an angry red colour, and that together with a generally higher sulphur content may have combined to give a brownish cast to the otherwise grey deposits?

After all, the Haynes plug diagram that I'm familiar with doesn't seem to have been updated for probably 40 years and is the same in every single manual I've seen it appear, so it could well be giving advice that's more applicable to the leaded (and, certainly for bikes, more "2 star" than "4 star") petrol, carburettor and "analogue" ignition days than the 21st century state of unleaded (and 95RON being essentially 4-star, ~98 the once-elusive 5-star...), direct injection and electronic ignition.

NB, isn't a "hotter" plug one that, perhaps unintuitively, *reduces* the combustion chamber temperature, by sinking more heat into the head (and thus plug and head end up hotter to the touch)? That's the way I was taught it anyway. NGK have a guide somewhere on their website which explains it, and the claim is that each step up or down the number scale (which differs in magnitude and direction per manufacturer... I *think* theirs is higher number = "cooler"?) causes a change in residual temperature to the tune of 150~200 celcius under otherwise identical conditions. Which has greater implications than just the colour of your sparkplug or the potential for the tip melting.

Too low and you're essentially running with a cold-started engine all the time, not burning the full fuel charge and/or encouraging the ECU to enrich the mixture (or the rider to put on more choke), losing power, wasting fuel, and potentially burning the cat if you have one fitted. Too high and, much like excessively leaning-out, it's a world of hurt - you're risking not only general overheating (boiling the coolant and/or burning oil) which could cause seizure or warping of major engine parts (with leaky or blown gaskets as just the thin end of the ultimate effect), chronic pinking and maybe melting the piston crown (and/or little-end bearing), which is itself almost certainly a one way ticket to seizure city.

It seems important to the extent that the advice I've seen for the CG at least is you should keep both a -8 and -9 rated plug in service rotation, and change them out as required at each minor service according to the season and/or your normal use of the bike (ie summer and/or long high speed runs vs winter and/or lots of pootling about in traffic), and additionally make a point of switching over if you know you're about to have a period of running counter to the usual state of affairs. And most places that sell plugs for the bike tend to have both of those available even for each of the other subtypes on offer (e.g. normal vs "iridium" vs multi-electrode...), and some even have the special offering of -7's.

Thing is, I can't remember which one was supposed to be used for which set of circumstances. And I've swapped between them before ahead of various types of riding and tried to fathom a difference between them, and I just can't. Maybe the bike really is just that low tuned and forgiving of abusive riding, forgotten maintenance and crap fuel that it simply doesn't make any real difference to it and the full range of temperatures produced by cruising at 30mph in winter with a "summer/fast" plug, through to absolutely caning the bike in a July heatwave with a "winter/slow" one, still doesn't exceed its safe (or even economic) upper or lower limits. Or maybe it's actually just bunk and engines built after the mid 70s simply don't care so much about the heat rating of the plug so long as it's not wildly out of spec? Particularly as both they and the plugs themselves are probably rather better made now?

I also have to admit ... I totally can't get my head around carburettor tuning. I've wondered whether mine was out somehow, looked at online guides about how to do it after finding there was nothing useful in the Haynes or the owner's manual... even had mine off and pulled it apart to see if I could figure what was what... looked at the half deconstructed mess on the kitchen table and tried to reconcile it with the scrappy printouts laid next to it ... gone "er"... and just put it back together in what I hoped was the right configuration and slapped it back in the bike. It seems to work. Maybe not perfectly, but good enough that the bike will usually start, even if it does take maybe 10 full seconds of cranking and a careful tickling of the throttle on full choke in winter (followed by a minutes or so of keeping steady and increasing throttle pressure until it finally revs up properly and I can drop to half choke and set off without the risk it'll conk out instead of idling or pulling away from a stand), which is not *quite* annoying enough to provoke further action.

There may be a way to twiddle it and make the bike start up better in the cold and run more powerfully and/or economically in summer, but it all looks like just so much impenetrable voodoo to me, with the idle jets and half and full throttle jets and needle positions and clip positions and float levels and slide cutouts and aaaaaaaaaaaaaaaaaaaaa. Not least because each successive guide seems to contradict the prior. I figure it's all a big conspiracy between the builders of old carburetted machines and their service agents, trying to make it deliberately too difficult for the home tinkerer to set their machine set up right unless they spend hours learning the secret magic spells, so that the more casual owner will just pay for a professional tune-up instead.

Give me a digital injection system and a software hackable EEPROM map any day... much more understandable. At rpm X, throttle position/load Y, and coolant temperature Z (or at least, those positions in the table closest to the measured values), fuel delivery rate needs to be N and ignition advance M. With a certain coefficient modifying those depending on the intake air temperature and density / mass flow rate, but that's usually a preset which doesn't need fiddled and we can just let the computer deal with it. Sometimes even the coolant temperature counts as such and we only actually need to deal with X and Y.

Hook vehicle up to computer, rev it up a few times at different throttle settings as it warms, see what telemetry data is fed back (put it on a dyno if you want to be fancy), tweak the mixture and advance anywhere it seems lacking, repeat the measurements - or sometimes you can actually make the changes "live" - see if things improve. If not, either revert, make the change bigger, or alter it the opposite way. All very quick, fine grained, relatively intuitive type adjustments, each only affecting a particular single set point in terms of throttle opening and revs so you can't stuff everything up with a single false move and have no idea how to fix it (instead of the rubik's cube of mutually interacting carb settings that govern behaviour across the entire rev/throttle range, or at best maybe a full 25% swathe of it, overlaid on all the others taking effect at the same place - and your only adjustment for temperature or air mass being the usually manually-adjusted choke), and applicable without even opening the bonnet in most cases, sometimes even whilst the engine's revving hard on a dyno, no need to lean over a running engine to tweak mixture screws, let alone take parts of the engine and the carb itself apart, or buy and swap out various new tiny, delicate jets and needles to effect changes. Or, indeed, as you can adjust the ignition at the same time, messing about with points and their separation or dwell angles, or tweaking the distributor back and forth to adjust the ultimate advance - which I even had to do on an otherwise electronically controlled system that had the hall sensor integrated to the dizzy (...and that's only for multicylinder systems, I've no idea how you'd do it on a single cylinder or a wasted-spark twin...).

That said, even the electronic adjustments are themselves something of a conspiratorial closed book, with it becoming a little more complicated to get access, layers of encryption and special programs (and, indeed, connectors and interfaces) needed with each new generation as the modder vs manufacturer (and government) arms race continues. Time was, if I wanted to give my hokey little Polo a couple more HP and smooth out that annoying lower-midrange emissions-reg flatspot, there was just a dude on any easily accessible owner forum who could hook you up with a replacement chip for twenty quid. Undo the ECU clips, carefully prise out the old ROM with the same screwdriver, push the new one in place (make sure it's the right way round!), close everything back up, and you're done. Nowadays it seems to take a specially prepared laptop (that never leaves the garage premises...) to hack the data in-place and the job costs at least ten times as much... [Le sigh]... Still, compared to a carb tune which could easily cost a significant fraction of that if not the full whack, have the vehicle off the road for longer, and end up rapidly drifting out afterwards anyway, I'd say things have got better.
7/11/2018 11:38:03 PM UTC

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