21 replies to this topic

[Davi926]

I recently installed a wide band air to fuel ratio sensor to figure out a few issues I've been having.
Reason why I installed it is If I use a colortune to get the idle to be a nice blue flame then the car is undrivable (goes off the scale lean).
So far the best I've gotten it is at idle I'll get between 10 and 11 to 1 which is rich, but when I put my foot down it leans out to about 15 to 1. Partial acceleration will see it in the 12 to 13 range and just keeping the speed on flat ground is the magic 14.7 to 1 ratio. But when I take my foot off the throttle it'll be in the 12 range again and will settle back into 10 at idle.
So far I've changed carbs (no difference).
A guide online suggested the dynamic timing might be off so using an advance timing gun it's now at 32 degrees at 4000rpm.
And changed the whole ignition system but nothing has made a change.
I'm running an aau needle and I have a new ADP needle which is the same ( almost identical to the aau) and that gives the same outcome.
From what I know I should be idling at around 14 to 1. Full throttle should be in the 12 range and cruising at 14. When throttling off it should be leaning out to 17 range.
Am I correct? It's running alright but still I know the fueling isn't right.

[Steve220]

Where is the lambda sensor positioned in the exhaust??

 

Yes, 10:1 is bad. It's bore-washing bad. So you may need to find a better needle and pot oil.

[Davi926]

Sensor is welded into the exhaust just as the downpipe bends horizontally under the car. Dash pot oil is 3 in 1.
I figured it may be the needle but aren't all needles profiled to get richer as you put your foot down? Seems mines reversed lol

[ACDodd]

Firstly well done for fitting a wideband, you have put yourself ahead of most Mini owners by wanting to correctly understand your engines fueling. It’s Simple you need the correct needle profile to fuel properly at all operating conditions. Together with the correct spring and damper oil of course, a custom profile needle is usually needed to achieve this….

https://youtu.be/rck8uPyaZKw

This is why I offer a mobile tuning service I make needles to suit people engine specs.


This is the usual effect I find when I come to tune minis, people turn the mixture screw rich to compensate for the needle being to lean. All this achieved is bore wash and ring wear.






Ac

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Edited by ACDodd, 02 June 2022 - 06:38 PM.

[Davi926]

What would people suggest I run in terms of needle? It's a hif38. Standard 998 engine. Has a k&n air filter and I believe a lcb manifold with just a back box.

[ACDodd]

Likely a custom one.

Ac

[Davi926]

Do you ever visit Wales?

[Davi926]

And yes it certainly was leaving black particles over my neighbors nice white car when they parked too close

[ACDodd]

I go international!

If you book me I will be there. Send a pm for more info.

Ac

[no66]

what type of afr kit did you install if I may ask?

[hazpalmer14]

what type of afr kit did you install if I may ask?

Aem do a sensor and gauge kit for about £160 which a lot of turbo/superchargered minis use

[Ethel]

Data logging would be handy, but you could always film the gauge(s) with your phone & do a commentary of how you're driving.

Some rough 'n ready carb theory to keep in mind when tinkering.

Carbs utilise the venturi effect - The pressure of a fluid passing through restriction drops as the rate of flow increases - because conservation of energy, or summet.

Stick a hole in the middle of the restriction (or venturi) and the effect can be used to suck up stuff (like petrol) and spray it through the hole (or jet) in to the airstream passing through the venturi. The size of the hole will give pretty accurate control of the mixture of petrol & air in constant operating conditions. However, it's quite handy to be able to control the speed your engine turns your car's wheels at, so we have throttles that also restrict the airflow & mess with that venturi-jet relationship.

An SU is a "variable venturi" carb which uses the difference in air pressure after the venturi, compared to atmospheric to make the dashpot piston rise and fall, altering the size of the gap, i.e. the venturi, above the jet and so give a more "constant depression" (an alternative name to variable venturi) over the jet. As the piston rises & falls that can also be utilised to alter the size of the jet with a tapered needle attached to it that goes inside the jet and makes it smaller as the piston gets lower.

From this we know:

The faster the airflow, the more fuel is sucked up, making the AFR richer unless the piston rises to adjust the venturi size and depression. That can also alter jet size - which we can control with the diameter of the needle.

We can also control the piston rising by altering the stiffness of the dashpot spring. Used to match the carb to the engine's air consumption, so the piston reaches its maximum height when the engine is flat out & we can use the full length of the needle for fuel metering.

Using thicker damper oil also slows the piston rising (AFR gets richer), but doesn't alter how far it will rise if the throttle is held steady - the oil viscosity can be used to further vary the AFR when the engine is accelerating.

[Lplus]

Sensor is welded into the exhaust just as the downpipe bends horizontally under the car. Dash pot oil is 3 in 1.
I figured it may be the needle but aren't all needles profiled to get richer as you put your foot down? Seems mines reversed lol

3 in 1 oil is too thin.  Burlen sell a specific oil, which I believe is SAE 20. 

[cal844]

Sensor is welded into the exhaust just as the downpipe bends horizontally under the car. Dash pot oil is 3 in 1.
I figured it may be the needle but aren't all needles profiled to get richer as you put your foot down? Seems mines reversed lol

3 in 1 oil is too thin. Burlen sell a specific oil, which I believe is SAE 20.

3in1 is an SAE 20 oil. Too thin. The OP needs to experiment with different oils in the dashpot to get it set correct for the carb. My bet is 20w50 engine oil as this is how the car came from the factory

[sonscar]

Dashpot oil is mainly for acceleration enrichment it does not take part in steady state.I find dinosaur engines like to idle best at around 12.5 to1..The thinner needle let's in more fuel as it rises out of the jet but also more air comes in.Custom tapering of the needle is as AC said is the correct answer.You could research the SU charts and buy more needles to try and you may find a reasonable compromise if you have enough patience and money,Enjoy tuning,Steve..