How is CO checked at MOT?

[rcx132]

My carbed car had very high CO when tested. I asked MOT tester if it was just at idle or under load as well. He said both.

I'm just wondering if this is just the idle needle jet. But if it is tested under load then it wouldn't just be the needle jet. But do they really test it under load during an MOT? I.e. at half and full throttle on a rolling road with a resistance load applied to the wheels?

 

[ms08]

At idle and at around 2500-3000rpm.

 

[steveo3002]

just been at idle with mine..depends what age etc

 

[rcx132]

At idle and at around 2500-3000rpm.

At 2500-3000 rpm but just sitting there, not moving, not on a rolling road? I.e. still within range of the needle jet.

 

[CompoSimmonite]

If it's jetted correctly for the engine then all you need to do is tune at tickover then mixture will be right thoughout the rev range. I've got a Gunsons Gastester and that allows me to often get the emissions of older cars within MOT levels but sometimes tester has to tweak the carb a little on more modern carb'd vehicles which have stricter levels.

Didn't the MOT tester offer to adjust as it's a couple of seconds job whilst it's hooked up to their equipment ?

 

[optima21]

I think mine has only been tested at idle, I set mine up using a vacuum gauge, just aim for the highest vacuum for a given rpm.....the higher the vacuum, the more power there is for the power stroke (or the better the mixture is being burnt).....

.....thats with an engine thats standard and used as a run around though

 

[rcx132]

Didn't the MOT tester offer to adjust as it's a couple of seconds job whilst it's hooked up to their equipment ?

Well he passed it.... but said it was at the limit, and often above the limit. Sparks plugs black as well when I got home. This was the cars first journey out.

Today I took it out for a fast run, then cut the engine and rolled into a car park. Plugs were nice colour. I think this is called "plug chopping". So I think the carb is ok at higher throttle openings.

Got home and while warm started adjusting the idle mixture needle. I wound it in until engine died, then wound it up, letting RPM rise. If I wind out further the RPM does not increase more but eventually engine dies. Tried to find a mid spot. Plugs are no longer going black. But engine is a little lumpy.

Ignition is all new, plugs are new, carb has been cleaned, fuel is new, filters new...

It does now respond to the idle screw. I think in my other post when I said it wasn't maybe the idle-up was on, or the butterfly was slightly open, so it wasn't running on the idle circuit, which is a separate circuit on this carb (downdraft carb).

If the plugs are a nice colour should I just leave things now and get on with something else? Or maybe ask if the MOT guy minds me bringing it in and setting the idle needle. I bring him lots of cars so he should be fine with it.

Hydrocarbons were very low and I think those are the indicator of a worn engine, so it does look like a needle issue.

Has anyone tried tuning using a lamda sensor? I have a spare narrowband one I could weld in a thread and screw it in, I've read that they are ok for basic tuning.

 

[optima21]

narrowband is too fine, it'll read either too rich or too lean (and will give a reading of 0V (lean) or 1.0V (rich)), when I set the airflow meter on my fuel injected engine (digifant ecu). I set it so that it just turns rich which (the lambda is not connected to the ECU), but still connected to a bargraph display) and then its 1.0V and leaner and it shows 0V. if you use a lambda for lambda for tuning, you'll need to allow it to heat up first, 3 or 4 wire ones have a heating element in them.

on my engine, the lambda isnt used by the ecu anyway, and then its only used to increase the economy of the engine at part and idle throttle settings, and cycles the fueling too rich and then too lean, to get an average fueling.

I'll then check that setup using a Vacuum gauge.

but wait until someone in the motor trade comes along and gives you a proper explanation

 

[rcx132]

narrowband is too fine, it'll read either too rich or too lean (and will give a reading of 0V (lean) or 1.0V (rich)), when I set the airflow meter on my fuel injected engine (digifant ecu). I set it so that it just turns rich which (the lambda is not connected to the ECU), but still connected to a bargraph display) and then its 1.0V and leaner and it shows 0V. if you use a lambda for lambda for tuning, you'll need to allow it to heat up first, 3 or 4 wire ones have a heating element in them.

on my engine, the lambda isnt used by the ecu anyway, and then its only used to increase the economy of the engine at part and idle throttle settings, and cycles the fueling too rich and then too lean, to get an average fueling.

I'll then check that setup using a Vacuum gauge.

but wait until someone in the motor trade comes along and gives you a proper explanation

If its just telling me too rich or too lean then that might work? Wind in the idle mixture needle, the Lamda will say it's lean, then wind out until the lambda says it's too rich, then wind back in a smidge.

 

[optima21]

yeah but you want it slightly rich at idle, think of a narrowband lamdba as a digital output (being either on or off), but think of a wideband lambda as an analog output (being a variable output) as it can show the who range of mixtures from way too lean to way too rich.