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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 tooErm...er...maybe 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.

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Reader's Comments

Trazymach84 said :-
Its normal for EFI bikes.
01/06/2016 08:45:59 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 lord...how old do I sound. Thanks :)
01/06/2016 09:09:54 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.
02/06/2016 07:35:55 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.
02/06/2016 09:37:20 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.
26/06/2016 21:57:25 UTC
Ren - The Ed said :-
Thanks Doug. Now you be gentle to that poor YBR and treat it right.
26/06/2016 22:29:20 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 :-)
27/06/2016 20:55:47 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...
28/06/2016 06:11:09 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 20:36:20 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 06:31:00 UTC

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