What’s the best way to clean up the exhaust tips?
WD40 and a kitchen scourer (plastic, not metal) works eventually but is there something else which will do a better job of loosening the deposits?
I’m thinking something like STP fuel system cleaner (with the rest going in the tank) but haven’t tried it yet.
I use this, works really well
Probably best not to use scourers as they may damage the chrome finish
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I use autoglyn engine cleaner. Gets them nice and shiny
Fine grade wire wool and elbow grease has always done the trick for me.
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Hmm, that also looks like it’s running a little bit rich, too much for the cat to clean up, if so it will soon be dirty again.
“Shiny and chrome! Witness”… I love that movie. 
Sorry, complete novice, what does running rich mean?
Don’t think they are chromed, just polished stainless steel so they’ll take a little scouring with wire wool.
Unburnt petrol making soot in the exhaust. Its a sign of something could be improved. Air cleaner clogged? Air leak? Timing? Bad fuel? Bad plugs? Dead cat? Burning oil? etc, etc,
Our cars have lean-burn engines, and the exhaust should be bare metal maybe with a hint of pale brown.
Fifteen year old original exhaust on MX-5 NC occasional heavy right foot.
Seven year old original exhaust on Mazda3 skyactive engine, even leaner burn (most of the time)
But then my 1996 Vectra exhaust looked like the Mazda3 exhaust all its 20 year life too.
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I’ve often wondered about that Richard but done nothing about it, but seeing what it should look like now I will.
Air filter is a clean K&N, doesn’t burn any oil between changes, cat was OK for last MOT pass but I’ll check emissions figures, fuel is BP or Shell with premium every 4th tank or so.
I’ve never checked plugs but I will now.
What would cause incorrect timing? Isn’t that set by the ECU mapping in a feedback loop?
Same with air leak. Wouldn’t that cause lean rather than rich running?
(Engine runs smooth and pulls well, never missed a beat.)
I use spray on oven cleaner foam to clean up the tip of my dirty diesel Merc.
Spray on ,leave for 15 mins, rince with hot water
Cone polishing goblet that attaches to a drill plus any sort of polish you have at hand
Autosol etc will take less time as it’s quite abrasive but any polish will give you a decent finish
Can be a bit messy as it flings contaminated polish around so use a slow setting (not the hammer setting!)
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If that exhaust pipe in your pic is wet and sticky, that is most likely from oil, and it looks like it has been doing it very slowly for quite a while to become more like tar.
However, yours is nowhere near as thick, sooty and tarry as a “low mileage” NC I walked away from at Car Giant nine years ago. I showed the obnoxiously pushy salesman the Vextra’s immaculate exhaust as an example of what it should look like.
There are many routes for oil to get through the combustion system, rings, valves, EGR, PCV breathers, etc. It doesn’t need to be much at all on some routes; for example if its being drawn in on the overrun when the engine is on maximum vacuum and no petrol being used to burn it off again.
There are lots of possibilities for a slight richness (and not necessarily oil burn), enough to still pass MOT, but adversely affecting mpg and clogging pipes, eg dirty MAF and air leaks.
If the MAF has become slightly contaminated with a faint hint of oil from your K&N, and then being sticky trapped some fine particles that squeezed through the K&N, ECU will think there is more air mass going through than reality (especially on low throttle) and richen up to balance out the mixture. Easy fix; clean the MAF with appropriate care.
Tiny air leaks in the exhaust can fool the o2 sensor into thinking it’s too lean, and again the mixture will richen up to compensate. This is more difficult, but a stethoscope can be very effective in locating such IF you know how and what to listen for; it’s easier to demonstrate than explain.
This forum has loads of expert advice on these topics, don’t just take my word for it, and it only needs a careful search to find useful solutions from them.
Have fun, after all it is an MX-5. More smiles per mile… 
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Many thanks for the detailed explanation Richard. I’ve been reading around the subject in other forum posts too.
The car was running cool when I first got it due to a failed thermostat and I imagine that may have had an effect as well. (Now replaced at 73,000 miles.)
The K&N is recent so I don’t think would be a factor, but I take on board what you say about the MAF sensor.
Is this just a personal thing or recommendation for every 4th tank?
That would also cause it to be rich (as DeadOrAlive asked).
Maybe the exhaust tip discolouration is historic from then?
Clean it up as suggested by the other posters and see what develops over the next few months.
Good luck.
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Just a personal thing, no real rationale behind it.
I’ve been trying to gauge if I’m getting any difference between them and think the car feels slightly more responsive on the premium fuel (although that may be psyhological) and noticed no real difference in MPG, although I don’t calculate this exactly.
Yep, I’ll do that, alhtough it does still smell rich at tickover and I’m getting 32-33 MPG with mixed driving and few occasions sitting in traffic. I’ve recently seen 37 MPG mentioned for an NC which I’m well off.
I’ve always assumed this was normal and it’s only your comment earlier has made me question it.
(Wooh! The sun’s just come out. I wasn’t expecting that.)
I’m getting around 37/38 mpg in my 06 plate 1.8. That is on a run over the south downs or on A roads . NO round town trips.
Hmm, stuck in traffic can ruin NC mpg.
The spreadsheet average on my NC for the last nine years and 67 refills is 37mpg. So yes it’s feasible. However I do try to avoid traffic (route, time of day etc.)
On a long motorway trip it’ll often do 42-43 for the tankful, and I tend to push on a bit too. It seems happiest when needle says 80, satnav 75.
Pootling around Northumberland and the Lakes with the MX-5 OC on a couple of trips a few years ago it surprised me and did around 47mpg on the two sets of two/three tankfuls.
Around town (but it’s rare I venture into traffic) or short wintery tips 32-36 for the tank. Mixed local motoring is always 36-39.
However, new tyres are needed for more correct figures. And I’ll be fitting 215/45 instead of the 205/45. Make and type undecided so far, priorities are low noise and super wet grip.
I don’t do enough mileage to bother about fuel economy, and I only look at the figures as a diagnostic for drift trends which might be symptoms of anything.
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Bar Keepers Friend works a treat!