How to get my emissions lower with carbs?

saitrix

saitrix

saitrix

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Joined
2 Jun 2003
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Nottingham
Hi,

Well I got double the allowed CO for my emissions, now I had my carbs rolling road tuned so I would rather not lose their settings. What can people suggest to do to help lower the CO? Already been told to remove my K&N filters, anything else?
 
Clearly they weren't tuned very well on the rolling road! If they are SU's then just weaken off the mixture and make a note of how many "flats" you have turned the brass nut by. Then you can bring your car back to it's full planet killing glory after the MOT :D
 
My colleague called round testers who he could slip a tenner to, job done. Saves peeing about.
 
If your car is hammering the emmisions test then they arent well tuned im affraid. The car is running far too rich on idle and should be re-tuned. Lesson one in classic cars dont trust so called experts they often screw your tuning. Propper tunned carbs should be perfect right through the rev range anyone can tune a car to its max power output thats easy. The hard part is playing with different needles etc to get it right through the rev range.

Drive to the mot station, lift the bonnet , turn the SUs 5 flats anti clockwise to lean the mixture stupidly, put it in for the test, if it still fails your problems arent carbs and could be elsewhere e.g. piston rings etc. If it passes drive 500m, lift the bonnet, turn them 5 flats clockwise, job done. But i would check out your tuning as too rich isnt good just as too lean isnt.
 
I know the needles are correct for my tune, smooth power throughout the rev range, looks like I will just have to have a play with my carbs.
 
Doesnt matter about smooth power, you will get smooth power from about 30 or 40 different needles, thing is you are running far too rich hence the emmissions failure, so the carbs arent working optimum at idle burning fuel and potentially damaging. Usually this wouldnt matter in a race car you actually want it tuned like that, but on a road car sitting in traffic etc you are always idleing.


The problem with using the adjuster nut at the bottom is it affects the needle through the rev range, so if you have decent power as you say, it could go too lean if you changed the setting for a leaner idle, hence always better to do the needle.


good site for needle choice, plots a graph of the needles
http://www.mintylamb.co.uk/suneedle/
 
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suggest to the mot tester/garage that you would like the mixture put down for the test then put back up afterwards and can he do this for an extra 20 quid :) :) :)
 
Yes but by the sounds of it the needle will be sorted to max power on the air fuel ratio. If it had been sorted through the rev range like it should have been done properly at the rolling road then it wouldnt fail the emisions test, as the air fuel ratio is measured by measuring the CO2 content of the exhaust gasses exactly the same as the MOT test, so your carbs dont have the right needle for idle due to the fact you failed!!!
 
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Mattius said:
If your car is hammering the emmisions test then they arent well tuned im affraid. The car is running far too rich on idle and should be re-tuned. Lesson one in classic cars dont trust so called experts they often screw your tuning. Propper tunned carbs should be perfect right through the rev range anyone can tune a car to its max power output thats easy. The hard part is playing with different needles etc to get it right through the rev range.

Drive to the mot station, lift the bonnet , turn the SUs 5 flats anti clockwise to lean the mixture stupidly, put it in for the test, if it still fails your problems arent carbs and could be elsewhere e.g. piston rings etc. If it passes drive 500m, lift the bonnet, turn them 5 flats clockwise, job done. But i would check out your tuning as too rich isnt good just as too lean isnt.
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With carbs being so good why has millions been wasted developing fuel injection systems? :) Carbs are generally only right over a very small amount of the rev range, everywhere else is an approximation.
 
Mattius said:
Yes but by the sounds of it the needle will be sorted to max power on the air fuel ratio. If it had been sorted through the rev range like it should have been done properly at the rolling road then it wouldnt fail the emisions test, as the air fuel ratio is measured by measuring the CO2 content of the exhaust gasses exactly the same as the MOT test, so your carbs dont have the right needle for idle due to the fact you failed!!!
Click to expand...

My AFR gauge reads AFR. I have absolutely no idea what the CO2 levels are when setting up the engine.
 
Your AFR meter reads lambda yeah? or does it give you 14.2? or the likes, in which case it is a reading of the CO2 content of the exhaust fumes, this content lets you determine if there is too much fuel in the system or not. If it reads in lambda then that is derived to help people understand air fuel ratios, lambda is basically the ideal combustion for petrol, or 13.9 AFR i think it is.


And in answer to the previous question, fuel injection is derived to be more accurate than carbs, it can also adjust its mixture on the fly rather than fixed mixture provided by carbs.
Carbs do not only cater for part of the rev range, they work through the whole range if you set them up properly. The whole rev range is easilly catered for if you know what your doing and have common sense and an afr meter.
 
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