Air/Fuel Ratio Variable

[oilypaws]

Hi!

I've been getting a bit of pinking at full throttle between 3,000 - 4,000 rpm.

I've twiddled with the timing a lot, but it wasn't effective until it got very retarded (1-2 deg BTDC at 450 RPM).

For a bit of fun, I bought an Air/Fuel ratio gauge and installed it. It uses an oxygen sensor which I fitted into the exhaust about 12 inches from the manifold. The gauge is a bit flashy with LED readout and things, so I hid it by my knee next to the steering column. It's a Bosh sensor and seems like a good quality product.

I'm wondering what readings I should expect while driving. I read one description which said the ratio should be 15 - 16.

Mine varies quite a lot depending on conditions. It is generally higher (=leaner) at higher RPM and smaller throttle. It gets lower (=Richer) as the RPM drops, and especially at wider throttle.

So when cruising on the motorway at 55 MPH, it is getting up to 17 or so (a bit lean?). If I put my foot down, it fluctuates (due to dashbot?) then drops to the 15-16 range. As the RPM drops (e.g. climbing a hill) I watch the ratio drop as well. In the 3,000 - 4,000 range at full throttle it will drop to around 13 (a bit rich?). Also, the idle mixture is around 11 when I set the mixture on the carb in the standard way.

Adjusting the mixture definitely changes what I see on the gauge in a predictable way, but the variations with throttle and RPM remain.

Is it normal for the mixture to lean out under small throttle / high RPM? Is this OK for the engine?

If I were to set the mixture using the gauge as a guide, what value should I look for and under what conditions?

Is this TMI... would I be better to throw the gauge away and not worry about it?

The car is actually starting and running very well, other then the small issue of pinking. I have set the timing back to the standard (4 deg BTDC at 450 PM). The pinking is very inconsistent. It usually only occurs when the engine is well warmed up, after driving at least 10 miles or so, and even then it doesn't always happen (but if it does,it's always between 3,000 - 4,000 RPM at full throttle). I keep thinking that it has gone, but then it comes back.

I also have a new distributor with electronic ignition. This made the car start much more easily and run more smoothly as the old one was quite worn. It made no difference to the pinking, though. It's supposed to be a replacement for the old one (45D I think...).

My carb came from a friend who is a Mini enthusiast. From the angle of the float bowl, it looks like a Minor carb, but the needle may be wrong. Would a wrong needle (e.g. with a slightly different profile) tend to cause the mixture to be very variable under changing conditions? I have a small collection of carb bits and spare needles so I could try a different one..

The car is a 62 4-door. The engine is a standard 948 (9M-U-L).

Great forum... thanks for any suggestions!

[chrisryder]

Certainly sounds like it could be the wrong needle in your carb. The ideal A/F ratio is 14.7:1. But I've no experience of what you'd actually get in the real world.

Get the right needle and see what effect it has on your readings!

Out of interest, where did you get this sensor kit, and how much was it?

[bmcecosse]

Your readings sound pretty much perfect to me - leaning out when 'on the cruise' and richer when accelerating. Does the pinking go away if you use premium fuel?? Just run the tank down and then try a couple of gallons. Any idea what mechanical advance is in the dizzy? And is the vacuum advance connected up? I assume it's working ok in the NEW dizzy - but what about the old one? Is the Mini carb larger bore - or just the same 1.25" bore as the old Minor carb?

[oilypaws]

Your readings sound pretty much perfect to me - leaning out when 'on the cruise' and richer when accelerating. Does the pinking go away if you use premium fuel?? Just run the tank down and then try a couple of gallons. Any idea what mechanical advance is in the dizzy? And is the vacuum advance connected up? I assume it's working ok in the NEW dizzy - but what about the old one? Is the Mini carb larger bore - or just the same 1.25" bore as the old Minor carb?

Ah! I was talking to someone else yesterday who also said that was normal (leaner while cruising). It's a standard size HS2 carb, and looks identical to the old one - but I think the number on the needle differed from the old carb. Do you know if they all have the same size jet (i.e. the bottom bit)? We decided the carb probably wasn't off a mini as the angle of the float chamber would be different (Minis have a steeper angle on the carb to fit it in I think). However, somebody might have changed the jet or needle.

The pinking is definitely worse with low octane petrol. I normally use "premium" which is 95 here, but the pump wouldn't reach the other day so I tried some 91 instead, and it made the pinking a lot more noticeable. I don't know the specs on the dizzy but it works (both vacuum and mechanical advance). The car ran a lot rougher on the old one, especially at low RPM, but I think that was because everything was worn and sloppy. It still basically worked.

[oilypaws]

Certainly sounds like it could be the wrong needle in your carb. The ideal A/F ratio is 14.7:1. But I've no experience of what you'd actually get in the real world.

Get the right needle and see what effect it has on your readings!

Out of interest, where did you get this sensor kit, and how much was it?

It's an Innovate Motorsports MTX-L. See here:

http://www.innovatemotorsports.com/products/MTXL.php

I got it for $250 NZ from the local auction site. I'm actually very impressed with the kit - it had really good installation instructions, and it has some handy features like a serial port so you can connect it to a laptop, and configurable analog outputs. All a bit high-tech for a Morris, but fun anyway.

[bmcecosse]

The readings you are getting are directly proportional to the needle profile - if you want it weaker at any point - use a thicker needle in that zone, and for richer - a thinner needle. Jets are all 90 thou in that size carb. Mini uses same single carb manifold and has same float bowl angle..... EXCEPT for twin Mini carbs which are indeed more steeply angled. Mixture can also be affected by height of petrol in the bowl, and strength of spring above the piston - and piston weight of course. The grade of oil in the damper assembly affects the 'rate of change' of mixture during acceleration - but not the static running mixture. Your pinking is maybe down to carbon build up in the combustion chambers. You could lightly try spraying some water through the inlet while revving the hot engine reasonably - keep it up for a few minutes - to see if it helps.

[MikeNash]

Dear Mr Oilypaws (Corr! where do these names come from?)

For me, this is one of the most interesting technical threads for a long time. I've often fancied such an Air/Fuel meter but at around £150 its a bit too steep for me; I envy you! From your first post I see that in the 55 mph/3000-4000 rpm range you're getting 17:1 at cruise and 13:1 at full throttle which carburettor textbooks put at about right. Note, that while 14:1 is the ideal chemical mix for complete combustion, in practice an over-richness will often give more power since the evaporative cooling in the intake raising the density and therefore also the weight of the inspired charge. And further, any unburnt fuel will cool the inners so inhibiting detonation, etc. (Anyone old enough to remember the exhaust smoke on take-off from DC6s/Stratocruisers/Constellations/Shackletons?)

When you got your pinking sorted, I wonder if might do some further tests? Perhaps some runs on a level road in calm conditions to give Air/Fuel ratios at a range of steady speeds, say 20 to 70 mph? Also some full throttle figures like wise? I'd love to know what you get, for its only with such equipment can we truly get real understanding of what's going on under the bonnet and therefore tune things properly.

Praps I should bite the bullet and get one too! Regards, Mike Nash.

[bmcecosse]

You can measure exhaust CO more cheaply Mike - which is a good guide to complete combustion. Oilypaws states the pinking only goes away when set to 1 to 2 degrees BTDC - that's correct! Leave it there - nothing wrong. Let the mech and vac advance adjust it as the revs rise. Sure - you could reduce the maximum mech advance inside the dizzy, and then increase the static advance - and 'maybe' it will go a little better, but I doubt the difference will be significant.And of course - you could spend money on an 'octane booster'

[MikeNash]

Alas Roy, Carbon Monoxide measurement is only useful with rich mixtures for when its weak the oxygen in the excess air converts it all to CO2. Also the only CO measurement device I know of (and got) is the Gunson thingy which requires a probe up the exhaust pipe and takes quite a while to settle. OK for initial settings at the tick-over but not much else.

At the moment I'm playing with fuel flow measurement again (by counting the SU electric fuel pump pulses) and trying to set up airflow measurement (on the bench with a vacuum cleaner suck device) by recording the pressure drop across the air cleaner intake nozzle. I'd like to measure the rise of the SU piston too but in the car that's easier said than done.

All in all Oilpaws's solution is much simpler! Regards, MikeN.

[bmcecosse]

I agree it will go to zero when weak - so just aim for the slightest trace of CO and it will be near enough right. I think there are portable units where you stick the probe - and fix it there - and then drive the car...

[MikeNash]

That's OK Roy for getting a max power but it doesn't help if you're trying for 16-17:I in the cruise mode!

[bmcecosse]

Just go on plug colours then.....

[philthehill]

Whilst the all singing and dancing with bells attached electronic mixture monitor may have its place but when it comes down to it the only real way to see how a Minor performs and to check whether the fuel mixture is right is by driving it! The Minor does not have an ECU or any other electronic engine management controls so what engine management there is is controlled by the setting of the ignition timing and setting of the air/fuel mixture of the carb (and the size of ones right boot) which are all manual/mechanical and initially set by the driver/mechanic. The mixture setting will always have to be a compromise with the 'A' series and so will the ignition timing as no two 'A' series work the same even if they look the same. Checking the colour of the plugs and the exhaust outlet will tell you just as much as any expensive electronic gadget.

[MikeNash]

Roy &Phil,
I think you're on the wrong wavelength to me. Of course, to get your engine running well and reliably the plug colour test is the guide - I've been using it on a variety of engines for over half a century! - but modern kit enables us to put figures to things and understand them better. No plug test will give you the Air/Fuel Ratio or how it varies as you drive.

Why do I want to know? Well, I just do; I'm a born experimenter (and was once paid to do it, too) and am eternally curious, and Oilpaws with his MTX-L Wideband kit has shown the way to actually understand a good deal more of what's going on under the bonnet. I hope he comes back and tells us more!
Regards, MikeN.

[bmcecosse]

You'll just have to fork out for the kit then.....but really - is it worth it? It would maybe be useful kit for a Club to own - the problem of course being the fitting of the sensor to many different cars...

[philthehill]

Whilst it is nice to know what is going on in the combustion chamber knowing does not necessarily make it run better! It does not appear to have helped solve oilypaws pinking problem even if he knows the air/fuel ratio at any rev range.
Therefore I will save my money and continue to endeavour to obtain maximum power out of my engine by the use of the seat of my pants and the colour of my plugs.