Technical black exhaust

[4paws]

For the last few months after long journeys my exhaust pipe at the rear has stayed jet black rather that the usual grey.

I thought the carb needed adjusting so the other week I set the carb screw using a tachometer on a multimeter as per Haynes and adjusted the idle to 850. I've done a couple of long journeys since 150+ motorway miles but no grey exhaust.

I've yet to check the sparks since adjusting the carb, but replaced them a a couple months back and had a look before doing the carb and the were somewhat coked black rather than brown as per the old ones.

Any suggestions what to try next to get my grey exhaust back?

 

[the mk1 kid]

Its obviously running rich which can be no bad thing.. I cant say i can picture a grey exhaust, al mine had been sooty black..?

Tom

 

[Dunc Uno Turbo]

as tom says, its running a little rich. as long as its not mega over fueling it will be fine! if it is mega overfueling, then u'll end up with bore wash

Dunc

 

[thepottleflump]

Surely a grey/metal exhaust would be a sign that the car is running weak also? I personally would expect the inside of the exhaust to be blackend - they usually are. Check your emmissions are correct and adjust the mixture accordingly or get a garage to do it for you. I think for a 1.0 fire the emissions should be between 1.5% - 2% CO2 although bigger/different engines may differ.

 

[Rob-mint-mk1-uno]

I agree with pottle flump... every exhaust i have ever seen is black inside, i have never seen a grey exhaust?? random.

 

[Chas Uno]

I get a grey exhaust only if I do a LONG journey (such as a 100 miles or so) on motorways, at a constant high speed with the engine running at optimum operating temperature for most of the trip. This is quite normal on older cars, especially those with a carburetor.

Round town and on shorter journeys the exhaust is more likely to be black, though not that bad.

However, it is rare that I get black spark plugs, and that is more of a sign of an over-rich mixture.

Have you changed the airfilter recently? A clogged air filter will reduce air flow and effectively richen the mixture. When was the last time the car was serviced?

 

[thepottleflump]

Whoops not something I've really paid much attention to, I know on my Alfa it is black but then that is a diesel

 

[Rob-mint-mk1-uno]

Wait for it to go bang bud then rip it's heart out and chuck a nice clean quick turbo lump in be a good excuse . far more fun....

 

[4paws]

Serviced the car couple months ago filters, oils, sparks...
You only get the grey coating visible in the tail pipe after a long 'motorway' journey - its the sign of a healthy engine + good for the exhaust. If you're just doing urban trips, short stuff then the exhaust stays black.

With all the cars I've owned and grown up with grey's been the exhaust colour after long trips. Its not an 'old car' phenomenum, although I've never owned a car with a cat.

My current uno used to turn grey, but now stays black. Its definately not a good sign after a long motorway cruise.

I'll check the colour of the sparks next week when I've got spare time.

 

[Chas Uno]

4paws, your best bet is to take the car to a garage or to someone that has an exhaust gas analyser. That way they can tell you what exactly is causing the problem, i.e. if the carb is running rich or if the engine is burning oil. Have you notice any change in oil consumption? Is the oil level staying at the same level or is it dropping quicker than normal? Also, is there any visible smoke out of the exhaust, especially when accelerating hard?

Keep at it and you'll get there in the end

 

[Goswell]

Hi

Just a point of interest. Here in South Africa we have changed all petrol from containing lead to totally lead free petrol. With the lead fuel and a correctly tuned engine, the exhaust would have a grey/white appearance, especially after highway use. Without the lead, totally black.

Regards


Terry

 

[Chas Uno]

Hi

Just a point of interest. Here in South Africa we have changed all petrol from containing lead to totally lead free petrol. With the lead fuel and a correctly tuned engine, the exhaust would have a grey/white appearance, especially after highway use. Without the lead, totally black.

Regards


Terry

Just thought that I ought to clarify, that there is no such thing as totally lead free petrol, at least not in the UK. That's why it's called UNLEADED rather than LEAD FREE, because it does indeed still have lead in it just in lower amounts compared with the old four and five star petrol.

Unless of course South Africa really does have petrol that is totally free of lead???

Also, it's not just the lead that makes the exhaust go black, it's the by product of hydrocarbons produced during the combustion process. These have been reduced by modern engine technology, but not eliminated.

That's as far as my knowledge goes with regards to petrol!

 

[alexGS]

I think Goswell raises the good point though that the exhaust pipe colour is very much affected by the petrol used. These days in NZ (we got rid of leaded fuel in 1996, years ahead of you lot ) we don't get grey exhausts, except (as Chas said) possibly on a very long trip where the small engine is working hard! And on the contrary, we've noticed an increase in exhaust blackness for engines running on the 96-octane unleaded fuels.

I would say it's not something to worry about; checking the spark plugs would be a more useful indicator of the engine's operation. This has also been a bit confusing since the advent of the new fuels: grey/brown deposits used to be the norm (from the lead compounds), but now a white, 'overheated' appearance is common with a black carbon buildup on the outer rim (probably, this explains where the carbon in the exhaust is coming from).

The root cause of the problem is that the octane boosters based on the benzene ring don't burn very cleanly. At least, that's my understanding and belief. (try burning some polystyrene for anecdotal evidence of what happens when you burn a benzene-ring compound )

The story seems to be different if you have a catalytic converter; that seems to 'after-burn' the carbon, leaving the tail pipe fairly clean for a long time.

If the spark plugs are uniformly black and 'dry', that suggests the engine is running too rich (black and 'wet' suggests an oil control problem; i.e. the engine is knackered).

If running too lean (e.g. due to blocked carburettor jet) the exhaust can also turn grey, but more exciting is the shower of sparks that appears occasionally at night, presumably as engine carbon is dislodged.

Carbon is sometimes annoying but very much an inherent part of our lives in an organic world...

-Alex

 

[Goswell]

Hi

Some food for thought. One of my other vehicles id a 1982 Honda 750K. I have had it since 84. About 2 years ago I pulled the head and replaced the two timing chains (Double overhead cams - 4 valves per cylinder) and cleaned and lapped all valves. Everything was in great shape and at this time we were still running leaded fuel and I had never had any problems with the bike. Shortly after changing to the 'Lead replacement' fuel, BANG!!!
An exhaust valver stuck open, the piston wacked it, broke it in half and smashed it through the piston. When I opened the motor, 5 of the 8 exhaust valves had to be driven out of the guides with a pin punch they were so tight. Once out, and wiped with a cloth, they slid back in with ease. They were sipply gummed up. All inlet were perfect. My thought is, lead is a lubricant, it used to assist in lubricating the valves. Now it is not there and the replacement additives are not as effective. Any thoughts?
It now concerns me with the Uno. Could this also happen?

Regards

Terry
C