Morris Minor Owners Club

Morning All!

Do any of you lot know what sort of readings I should get from a 1275 engine?
I know its dependant on carb and whatnot but I'm after a ball park 'ideal' figure to aim for.

I'm borrowing a emissions probe machine from a chap at work and thought I could get the fuelling pretty spot on with it. Use it a bit like an AFR meter for setting up at idle.

Anyone?

cheers
jon

No worries - found out now.

2 - 2.5

That and much more useful A-series info from the Sprite Tech pages:
http://www.austin-healeys.com/spritespecs.htm

I always figure the idle is spot on when it's spot on!

Tweaking the mixture nut till the tickover is fastest and smoothest does the job for me, though you could do that and wind it lean a flat for slightly better mpg.

Fortunately emissions are not important for MOT on any Minor. The figures quoted are only for idle - and honestly - the engine only idles for a tiny % of it's running time - especially if you think in terms of engine revs turned rather than time!! Spark plug readings will give you the true mixture efficiency. It could be correct at idle - and miles out when running, and of course vice versa.

Fortunately emissions are not important for MOT on any Minor

If the examiner can see across the workshop at the end of the test, it has passed the emissions test.

The figures quoted are only for idle - and honestly - the engine only idles for a tiny % of it's running time - especially if you think in terms of engine revs turned rather than time!!

Ah. but thats the joy of an SU carb. Getting it right for idle should make it right throughout the rev range - provided the correct needle is fitted.

Spark plug readings will give you the true mixture efficiency

True - but in my case these were misleading. According to the plugs it was spot on yet the figure I was getting went off the scale at 6! I've now sorted it at 2.5 and it idles lovely.
Previously I'd used the 'piston lift' test to set it, according to that it had been spot on.

Anyhow, the proof will be in the driving. I'll whip the plugs out next week and have a quicknosey at them too.
However, this morning I had to use a bit of choke - first time I've used it since fitting the HS4 carb last year! This has to be a good sign.

FWIW here are the results I get with an exhaust gas analyser.

If mixture set with carb lifting pin idle Co 7.4 at 800 RPM, drops to 5.4 at 2,000 RPM

If mixture set for best idle speed Co 3.5 at 800 RPM dropping to 2.8 at 2,000 RPM

For best ecconomy set idle Co to 3.5 to 4.5 this is still a little on the rich side, but ensures sufficient valve cooling.

If no exhaust gas analyser available, best method is to set mixture with the carb lifting pin, so that 0.8mm piston lift gives no change in engine speed, then turn jet adjuster nut one flat clockwise viewed from underneath. This will give 4.0 Co or very close if engine in good health and carb not worn.

Dunketh - virtually ALL SU needles have exactly the same profile in the 'idling' section - so setting the idle mixture is just that - the running mix can be miles out! What would be worth doing is full throttle hill climbing trials with exhaust gas analyser connected - followed by part throttle constant speed running, and from the results a better profile needle could be gauged.

bmcecosse wrote:Dunketh - virtually ALL SU needles have exactly the same profile in the 'idling' section - so setting the idle mixture is just that - the running mix can be miles out!

You'd think, but after much needle swapping to get my HIF38 working through the range (it's still not, I'm thinking of going even harder on the spring) I found a definite difference in idle between certain needles. Makes no sense when as you say the idle profile is near as dammit the same on all of them, but it was certainly the case!

The needles tend to wear there (idle position) - where it's rubbing against the jet - and that's the bit that gets most use! I wouldn't go heavy on the spring - just want heavy enough to get full open at full revs under load! What air cleaner are you using ? Have to say I can only get mine to run cleanly with NO filter - not even the casing! Still to try the fancy K&N copy I bought from ebay.

These are new needles! Starting to get just a touch pricey for no improvement

The carb came with a K&N cone - I reckon if I stuck something more restrictive on I'd be fine, but that seems backwards when looking for more performance! It just feels like it's too lean on the move, but too rich at idle. I can't be sure there's no air leaks though as the inlet flange is thinner than the exhaust and my quickly bodged spacers might not be doing the job. Next trick is to make some inserts to go in the pleasts of the filter to try and fine tune the amount of air being drawn in. Bodgetastic I know, but if it makes things run clean and smoothly I don't care!

As for setting a mixture to book figures, it has to be remembered BMC/ BL would've been looking to meet various emissions levels and desired miles per gallon. That's why I far prefer to set things to where the car *feels* the best rather than worry too much about what numbers some gizmo is telling me

Hello all,

If you are serious about mixture tuning, it is feasable to fit an O2 sensor in the manifold or down pipe, (close to the engine) and use a multimeter to monitor mixture under load.

Packedup, I don't know if you have heard of Kas Kastner, a legendary American Triumph tuner. In one of his books he shows a little machine he (simply) made to make S.U. needles. I guess he needed a lot of them?

Alec

It is very tempting to just fit an AFR meter. Trouble is I'd have to drill and tap the downpipe to fit the sensor boss. I think it'd be well worth while though - if I didnt have a thousand and one better things to do.

Yes - if you start with a fat needle (ie too weak) you can steadily file it down until it's just right! But much patience required.
What needle did you start with dunketh - the Mini lads reckon AAA is a fair needle on a 998 engine which is not too different to the 1098. I can't rmember what's in my HIF 38 (but no filter) at the moment - first dry day I will have a look for you!

dunketh wrote:It is very tempting to just fit an AFR meter. Trouble is I'd have to drill and tap the downpipe to fit the sensor boss. I think it'd be well worth while though - if I didnt have a thousand and one better things to do.

The easy bit is welding a nut to the downpipe and sticking a lambda in it - It's the wiring it up to something useful that puts me off!

You could wire it up to a servo or stepper motor in the carb to adjust the mixture strength by raising and lowering the needle maybe? That's if you enjoy completely pointless exercises...

What needle did you start with dunketh - the Mini lads reckon AAA is a fair needle on a 998 engine which is not too different to the 1098.

You've got me there!
I dont remember. It was whatever WinSU suggested for a 1275 with HS4. It runs spot on and never has any flat spots so I guess its correct - I'll have to look at the receipt or whip it out and have a look.

So - it ran ok on the HS4, and now you have changed to a HIF38?
Some say the old style plain shank needle can be fitted into the little holder for the HIF carbs. Not tried this myself! AAA iwould be an HIF type needle.

I'm still on an HS4.
Used to be on an HS2 but I went bigger and couldn't afford an HIF44.

I dont suffer any surge issus so I can't see any benefit in going to an HIF equivalent.

Sorry Dunketh - it was packedup came in with HIF38 and confused me. Easily done these days i'm afraid ! So - you are on HS4 and looking for a needle. Wizard suggests Red spring and DZ needle for normal, then BQ or BP for higher tune, going to No 7 needle for top tune.