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MG Midget and Sprite Technical - Carb mixture

Am I right in my following thinking?

Taking too long to set the carb mixture means the car gets too warm and the mixture correct at that point but too lean in later road use. So that when the car is driven and gets warmer at slower speeds and stops/starts it can have 'hesitations' because the mixture is now too rich.

I'm bad at mixing up opposites and reverses, left/right, up/down, cw/ccw, ect.. My wife often says things like "no, the other left" (only once today though).
Nigel Atkins

Nigel
Mixture should be set up at normal running temp-
IF you're setting it by lifting the piston, then, if the engine is too cold, then the mixture will be too rich when the motor warms up
Setting it when the motor is slightly over warm shouldn't effect mixture
Setting mixture with a gas analyser or co meter is best as the reading shouldn't vary over a wider temp. range

IF your car has a HIF carb fitted then it's important to have it up to temp as the carb senses fuel temperature as well

willy
William Revit

I've never had a problem with the car getting too hot whilst tuning the carbs. It will get to a good working temperature, but that's what you want. But never overheated.

But if using a gas analyser then you have to stop and give the engine a good few revs to clear out it's pipes every couple of minutes, before you can continue. And then the gunson analyser that I have needs calibrating and stabilising. It's all a bit of a nuisance and not very convincing that it's accurate at the end. A road test and how does the car feel to drive seems to me a more reliable way once you've balanced the carbs and done the basics.
GuyW

HS2s, lifting pins only, at normal running temperature.

Previously I had it OK but slight hesitation at 6,000 revs on road test which I thought I'd get rid of with a tweek, bad mistake. I never normally go above 5,500 so I don't know why I bothered but like an idiot I did (vanity and self delusion I expect).

Sitting outside my car goes from warmed up to getting warmer quite quickly and everytime I go out to try setting the carbs the clouds part, wind drops and the sun shines, to go back in when I do.

I've tried to allow for putting the (K&N) filters back on and then ignoring this. AFAIK I got it right yesterday but the test drive had 'hesitations' when the car was warmer at lower speeds.

Perhaps my K&Ns are clogged up but I can't see it or understand why.

Whilst trying to set the carbs there's a wide band of sameness in engine and exhaust sounds and no visible emissions from the exhaust at all so I'm happy to leave it anywhere set up for reasonable running and get a proper expert to properly set it up for proper performance but I want it running reasonably until I can get the car into the expert.
Nigel Atkins

<<my car goes from warmed up to getting warmer quite quickly >>

Perhaps your cooling system needs a good flush through. Somewhere I have some very detailed notes that someone here kindly sent on how best to do this flushing and cleaning to overcome poor circulation, overheating and heater problems.
GuyW

Also, if it's going ok everywhere else but going off song at 6000 it 'could' be more of an actual fuel supply issue------------partially blocked fuel filter--?
or plug gaps too wide
or over advanced ign. timing
or ign. points closed down
or,or,or

So, why does your car get hot quick, does it have it's original fan or have you 'improved' it by going lectric

willy
William Revit

Oh Willy, I think Nigel has gone all electronic, so no points to adjust.
But maybe his butterflies are not fully opening.
GuyW

Cigar to Guy for the 6,000 issue - I forgot the butterflies don't fully open, I'd never noticed before I replaced the linkages, it was on my to-look-at list. Sod my poor memory, I could have left well alone, I had it running (reasonably?) well apart from that.

The 6,000 was now in my mind as not to worry about as, I mistyped, I only usually go to 5,000 with the odd stray higher.

The cooling system perhaps isn't as clean as it once was but seems to be working very well, the engine also quickly cools.

Willy you're right I've changed to electric fan but can set it to come on at normal running needle point on the (new) gauge.

I was considering to not even bother doing much of a service this year as the car has been used so little in the last year (and two and three years) but I couldn't bring myself to do it and decided to go the other way and give a fuller replacement service to the engine and give it a bloody good thrashing whenever I could to justify the expense (not monetary) and (literally) pain it causes me.

Plugs are gapped at 0.028" as previously discovered best setting for my car (of course that might have changed with other alterations).

Ignition timing is where it was when it was running very well before, if anything perhaps it could be slightly advanced now which I was going to look at once I'd got a running setting to reinstalled carbs (and then of course I'd need to perhaps fiddle with the carbs again).

Tappets are set (and not even by me), the dissy is wonderfully easy fully electronic 123 set previously to correct setting confirmed by expert rolling road tuning, on a few times now.

New items fitted, quality dissy cap, (hopefully too with) rotor, quality HT leads only a few years old. I'm just now fitting a new 'Lucas' without the label coil (looks to be of oriental quality of build) but I've also just took of the 2008 'Lucas' I gave to my mate who lent it back to me to swap out with my existing 'Lucas' coil, just in case and easy and available.

I've just turned both nuts half a flat to "windscreen weak" with filters on and not bothering to readjust balance to see if things improve or get worse - back to real basics!

I'm kicking myself about the butterflies as I was going to put it on my list to ask a mate about.

Sorry I didn't say before, thank you for your replies and help
Nigel Atkins

The cooling system comment was a lure, Nigel. Sadly you didn't bite, but I am pretty sure you realised and stored up your response for a more opportune moment! ;-)
GuyW

Mix - bin the Gunson type things - useless. Back to basics. The colour of the plugs is a good start, and from there, how does the car perform? If it overheats, then possibly due to a weak mix, if it smokes, bogs down or uses too much fuel then too rich. I find a good throttle response throughout the rev range up my local steep hill is a good indicator, and then I tend to run a tad rich above that.

Carbs/mix are the very last thing to fiddle with. Ensure everything else is bob on before tinkering.
Oggers

Guy,
I realised your lure was a response to an earlier jibe I made in another thread but I did/do have a prepared response which I hope to throw in given the opportunity, even though I shouldn't as you helped me realised what I'd forgot. But I'll probably forget about that reponse anyway. :)

Also your detailed notes maybe a previous version and need updating, do let me know. :)
Nigel Atkins

Earlier Jibe? If there was I have forgotten or missed it.
I think my copy of the enema for cooling systems must be an early version.It seems to be inscribed on tablets of stone.
GuyW

Willy,
with the electric cooling fan do bear in mind that although, IIRC, I live in one of the most protect, driest counties in England it can get very hot and very cold (by England standards) and my car sits outside 365(/6).

Air temperature (which doesn't fully reflect the heat and cold) can go from say -10C to say +35C, the electric fan is particularly useful when the weather is in the colder end of the range.

Also today I was able to delay the fan cutting in so that at many points the car was running warmer than normal to give the carb mixture a greater test, 24C today.

Tomorrow I hope to do the same testing as 26C is forecast, Tuesday 25C - all too hot for me if the sun is out.


Nigel Atkins

Guy if your version has any spelling mistakes, typos or poor grammar then it must be a very early version. Unfortunately MS Word doesn't tell me how many previous version but it was last modified 09/08/2019 15:38 (but could have easily been a correct to a previous correction).

If you have an early version hold on to it as I believe they're becoming very collectable by those in the pycriatratic field for study of induced altered states.
Nigel Atkins

Mark,
blimey we singing from just about the same 'him' sheets, unusual for us.

None of this carb fiddling was done by choice and previous fiddling is usually restricted to a quick tweeking and test runs.

In the last year or so I have used a airflow meter to balance and wished I'd got one years back.

I'm only after reasonable running from my faffing about, for what I want out of the car only a rolling road tuning will fully get there (for however long it remains). I'm not after ego figures or performance peaks but optimum regular reliable road use performance (which obviously includes fuel economy within pedal pressure) with what's available.

I'll put more info later of why things weren't as straightforward as they should have been, even allowing for an idiot like me doing it.

Nigel Atkins

I have never found that the lifting pin works on my HIF44. The revs always drop. Plug colour is the best guide for me. I would like to try a CO meter. I see they are available for about £20. Are these cheap ones any good?

Les
L B Rose

Nigel

It is a question of working methodically more than anything else. Read the manual is a good rule of thumb. Do not be tempted to fiddle unless you are absolutely certain, and if fiddling does not work, then you must revert whatever you fiddled with back to the original setting before fiddling with something else.

You do not need a rolling road either if all you desire is decent running from a standard set up. Just follow the basics of setting up the engine/timing/vv clearances/carbs etc. correctly and you should be fine. Of course, take a good look at all hoses, wiring connectors, plugs etc beforehand to assess for air leaks, poor earth, high resistance, fouling etc.





Oggers

Thanks Les, I too find lifting the pin very unsatisfactory, who are these people that can feel or visually judge lifting two odds shapes 0.8 or 1mm apart!? Or hear much different in the sound of the idle?

I don't like trusting any electronic equipment that is cheap or borrowed as I've no idea of its accuracy or reliability and I have no knowledge of allowing for this so if there's a problem I don't know if it's the equipment or me that's at fault or both.

I've been lent various testing equipment for various jobs and not just on cars that I've had to repair or not sure about reliability including a timing gun - but I've no interest in buying the equipment as I'd so rarely use it to justify the ownership or be sure of its reliability and calibration after years of storage and non-use.

Looking at the plugs at least gives an idea of the extreme ends of the mixture range but only if done appropriately.

The additive I use gives a range of samonish colours to the plugs, only way to really do it properly is to replace with new plugs at each test!

27C here today so I hope to set the electric cooling fan to have the engine running normal than usual plus the ambient temperature as a push test of my current settings. I'm going to see if I have a 0.8 or 1mm blade on my new (cheap Blue Spot) set of feeler gauges.
Nigel Atkins

The lifting pins are your friends
So what happens when you lift yours
William Revit

Mark,
totally agree, and it's what I did. The problems was my memory, forgetting the butterflies don't fully open (another thing on the to-do-list) and that I had an intermittent that I guessed at but thought I'd resolved but Sod's Law wasn't playing up later when checking for it but was there when setting up or at road testing (sticking jet).

I want more than decent running as my car is modestly off standard and with the vast costs (not just monetary but I could have run a fleet of MX-5s) and the very little use I get out of the car I expect to get good performance out of it when I can and want, its a sportscar after all. I sometimes pottle around in it but I want to know it will, within its limitations, go well. This is my 4th 'MG' and I've owned quicker, faster and more powerful sportscars so I know what I expect and for the carbs can only be set up by a rolling road tuner who knows what they're doing.

I've often put about avoiding carb fiddling, I'm against it, get them set properly and then leave them alone, I avoid all unnecessary carb fiddling but 'events' mean they can't be left alone for a fraction of the period I'd prefer.

I thought I'd made it very clear, repeatedly, that I ONLY enjoy driving the car (when it's going well) and certainly not working on my car, never, ever.
Nigel Atkins

I suspect the lifting pin problem is from lifting it too far. It only needs a very slight rise for the test. Usually works for me, though not when the needle valves still persist in leaking!
GuyW

Willy,
they are extremely poor friends then!

When I attempt to lift them what I think is 0.8-1.0mm as far as I can tell, to use a technical term, bugger-all. I'm sure others can judge 0.8-1.0mm and might hear any difference but that aint not I.

Guy,
I take your point but I allowed for that and it wasn't the problem(s) (plural), main problem was/is certainly me but I will give myself a little leiaway and cite Sod's Law.

Both,
I think my poke and hope method has worked but will know for sure tonight hopefully, again to be fair to myself I have been very methodical (apart from memory loss and missing the actual problem) and have three times returned to base setting and built back up from them, confirming my settings where at least grouped. I've used the Vernier once and counting flats twice. IIRC three different methods of getting the set up.

I'm now booking/ed in with Peter Burgess for his expert rolling road tune-up and reasonably confident of making the 75 mile journey there - but no doubt if it's not the carbs the car will find another item to play me up with going up. I'll get Peter to swear that my settings were so near that he could make no great improvements and despite the 7% drop in figures with his dyno's new software my car is putting out more monkeypowers and more, and more level line of bananas than ever before and that I can almost hold a spanner.


Nigel Atkins

For those of little faith -
I've just been out in shade air temperature of 27C ("feels like" 30C air temp), I've no idea of reflected tarmac and concrete temperatures. I and the car were not in the shade so had the 'benefit' of the extra heat from the sun which felt considerable.

I dressed in s/h F1 pit apparel as I knew this would help with technical competence.

So I got a 0.889mm feeler gauge blade to allow for the fact I don't currently have access to a calibrated and certificated measuring instrument hoping it would at least be within the 0.8-1.0mm SU variously requirement (0.035" on the blade for you traditionalist).

This I thought to use initially to more accurately ascertain visually (with my poor quality eyesight) what the piston looks like when raised about 1mm as I was previously going on my memory of ruler markings.

But I then thought why not use the blade through the air inlet to directly lift the piston (between 0.8-1mm).

This didn't work as my new 32 blade Blue Spot, Americain (Chinese made?) feeler blade was too wide and interfered with the mixture.

On to trying my Halfords feeler gauge (of unknown manufacture origin) 0.025" and 0.010" blades combined, again I hope these and the gap between them would still meet SU requirements.

These I could just insert without causing too much blockage to air/fuel mixture package.

I also went a quarter flat 'radiator rich' on both carbs (Guy you can have that reminder method for free, no don't thank me).

So, what differences did I notice - bugger-all.

More farting about for nothing.

If you say there must have been a noticeable change of some sort then you'll have to come here to show me.

Now the idle doesn't 'sound' as good today as yesterday, perhaps it is, perhaps it's not, I don't know, I'm dizzy from going round in circles and if I also get over my sunstroke I'll test drive tonight.
Nigel Atkins

Nigel, you are making better progress than me. It must be the heat down your way. Here it's been building all day and has just topped out at a tropical 21C.

I don't understand your "quarter flat radiator rich" comment, free or otherwise.

Mine is going backwards. No, not in reverse, but after starting and running the car for a few minutes that rear carb is now overflowing again. Not quite as bad but a definite flow up out of the jet with ignition on and engine not running. Pump ticking at 22seconds.

Engine still popping, hesitating and missing and will only keep going if I play tunes on the throttle and choke cables. I don't know why because when I first started this engine a couple of weeks ago it ran quite sweetly from the off.
GuyW

Standard comment - rotor arm, condenser !?

What else have You done ?

R.
richard b

Well my problems were from missing basics really, the plugs were the best indicator of things being way out but only under certain circumstances otherwise they can possibly put you off the scent.

What about swapping the lid and all internal parts from rear 1275 carbs which at least worked when last used.

I have a raised idle to start with and warm up using fast idle even then might have to dance on then pedal to keep things going until the engine warms.

You're so lucky to have an old British designed pump that reminds you its working rather than keeping quiet.

You missed the quotation marks - 'radiator rich'.
Nigel Atkins

Hi Rich,
if you mean me I've listed it somewhere but it's definitely me with the carbs. If I'd had remembered the butterflies didn't fully open all would have been set good enough a good while back but instead it's been going round in circles, with an added in intermittent jet sticking, until I've disappeared into an orifice. Which many think is my natural stance.
Nigel Atkins

I didn't miss the quote marks Nigel, just didn't bother inserting them when replying. I still don't understand. What does 'radiator rich' mean?

And, Richard. I fitted new points, condenser, rotor and dizzy cap. All new parts from simonbb (he of electronic ignition device, though I don't use one), And I fitted a new LT wire from points to coil. I have tried several rotor arms none of which improves. I will try putting the old Lucas dizzy cap back on next. New one looks Ok, but they do say looks aren't everything.
GuyW

Nearly 80 posts on the simple basic task of setting up the SU carbs, there'll be tears from some viewers.

At the risk of insult - HT leads in correct order?

When I changed the coil yet again I thought as I walked off the leads look a bit odd, I'd left the coil without its HT lead connected.

Turning the flats towards the radiator . . . and 'windscreen weak'.

Now the sun has just about dropped it feels like the temperature has too in the shade but it's still actually 23.5C ("feels like" 26C) in the shade, proves how much the sun adds.

Nigel Atkins

<<flats towards the radiator . . . and 'windscreen weak'>>
Ahhhh !!

I would struggle with that anyway as when a flat goes towards the rad, there is another round the other side going the opposite direction!

Cables are the right way, but you are not the first to suggest that, and TBH, it is behaving much like that so I will check again when I swap my Lucas cap back in again.
There is another thing, but I will check it out first before giving any further details.
GuyW

Yes I supposed being tall you lean across the engine to stretch your back and the carbs face the opposite way.
Nigel Atkins

Guy
It will never be right until you stop that carb. flooding up

Nigel
Forget about the feeler gauges and all that crap
Use the lifting pins---lift a pin till you feel it touch the piston, if you have the air cleaner off you can have a little peep through there so as to get a feel for when it touches
now- start slowly lifting the piston, you should hear a slight rise in engine speed, not much,if idle is say 600 it should go to 620-ish ,but a slight rise at around 1mm lift, then continue slowly lifting and that rise will drop back and then as you reach full lift of the pin idle should drop off again and possibly misfire/run rough
IF it works like this your idle mixture is correct IF your revs don't pick up at all and the pin lifting causes the idle to drop off lifting the pin further-then it is too lean
If you lift the pin and the revs increase and lifting further stays increased further up towards the limit of the pin then you are too rich

To use the 1mm setup procedure you need a very acurate tacho-------Poke a piece of 1mm wire (nothing wide to block air flow)between the piston and the bridge (1mm piston lift) and then watching the tacho ,adjust each carb mixture to get the maximum idle speed mixture setting point ,then remove the wires from the bridge and then your lifting pins will operate as they should and the mixture will be correct

Looking up towards the bottom of the carb. lean is clockwise rich is anticlock
William Revit

I've never set the mixture with the lifting pins. I prefer to get the engine to temperature and raise or lower the jets till I get the best idle. Usually there is a sweet spot where one or two flats this way or that makes no difference - and this will coincide with the highest rpm and vacuum reading. This establishes the best idle, and setting max advance to 30 degrees a safe timing limit. If your engine is modified the shape of the fuelling and timing needs to be addressed on the rolling road. Nigel l use Rod Taylor in Southam - worth a visit if you haven't tried him.
f pollock

Willy, you are right if course. For mine, the flooding and fuel level needs to be sorted first. I've not started on any adjustments yet. I have ordered Vitron tipped valves. Although steel tipped ones wear into ridges, these were brand new and there shouldn't have been a problem.

I earlier fitted new ignition parts and set static timing but that was just part of the rebuild, not a final setting up.
It's a bit confused with two carb threads running simultaneously.
GuyW

Hi Willy,
please don't take offence, I value your advice and I'm not getting at you. I like the idea of 1mm bits of wire as I like all simple straightforward stuff that provides the least of amount of farting about on cars. Although I don't own a hardware store with lots of various gauges of wire I'm sure I could ruttle around and find odd bits of various wires, borrow a Vernier and check its 1mm (subject to the Vernier being accurate to that point). I'd also have to find someone with a trustworthy accurate instrument to use as an accurate tacho.

England is not how you imagine it, well not my part anyway, you can't nip into places and get stuff done for a beer or borrow professional tools or equipment unless you know the person really well and then they'd be reluctant to lend their expensive work equipment to an idiot, would you?

I've tried three different methods and I think unless you're very experienced or just naturally very good at judging pushing a spring loaded pin to lift against a resistance just 1mm, for me, it's bollocks. You and other may be able to do it but I bet if I was able to measure what most of us think is this 1mm lift it wouldn't.

I was putting up a bit of a lighthearted (or pisstake if prefered) post with the three decimal place measurement feeler gauge blades (but that is what's marked on them) about judging 1mm by eye or feel for those not experienced. I did use the blades to give me an idea of what the piston lifted 1mm would look like but I couldn't tell even with the then and I'd not be able to replicate it by eye.

My car has never idled at 600rpm, an indicated 800rpm is about the lowest I can remember, I also remember the tach is out by 100 at certain point(s) but not where.

I'm sure if you were here you'd have in done in a couple of minutes - but I'd make you prove you could lift the pins exactly 1mm without any aids. :)
Nigel Atkins

Fergus,
the best idle method was my preference and certainly went across a wide range but the instructions I followed was to go beyond this and slowly back to the edge of the drop off. If I was doing it again I'd just stick with anywhere in the fastest idle range and forget the just at the edge bit and do road tests and later fine tuning instead - or just leave it there as I know everything is clean and lots of new working parts elsewhere on the ignition.

My plan was to get the carbs back on and set up for reasonable running then perhaps adjust the timing (by twisting the 123 fully electronic dissy) and reset mixture as required. But all only to get reasonable running to get it to an expert for rolling road tuning.

I've always put on the BBS the need for expert rolling road tuning to get the best performance set up, especially needed if the car is off-standard but also good on a standard car but most are either happy with what they've got which is fine or too tight or too vain to see the extra value.

With my previous MGs I used to go to Trevor Shaw at Daventry, proper old school, but he died. I was recommended an expert A-series rolling road tuner and he wasn't at all good when I took the midget to him and had ego gauges that would make any paper print off not worth the paper unless you wanted ego figures. I then went to a certain Mr. Peter Burgess and found his expertise and outlook to be much more suited to my needs (no ego figures or runs).

The car would have gone to Peter this August but life got in the way. My goal is always to have an expert do the proper tuning and for me to leave the carbs alone as much as possible but the car and others have generally scuppered these plans.

The car will be going to Peter's so I just want to get it running reasonably well until then.
Nigel Atkins

Guy,
sorry about the two threads, initially I only want to know what was asked in the original post but things develop. I feel if anyone posts on a thread, certainly one I've started, that I at least ought to acknowledge that and reply.

Nigel Atkins

No need to apologize to me Nigel! Your thread was for a different reason, but inevitably the issues overlap and things get a bit confused!
GuyW

All good Nigel-
If you are going to set it up by fastest idle method, for me i'd give 'em half a flat towards rich when you've finished, you'll get a more reliable idle over a wider temp range-
If you were to leave it without this little tweek you'll find that as soon as it cools off a bit the idle will change/drop a bit
cheers
willy
William Revit

Any tips on how to balance the two carburettor threads? One seems to be running faster than the other!
M Wood

I think one may have an air leak and the other is definitely running a bit rich. Could try throttling one perhaps?
GuyW

Willy,
previously I was allowing a little weak to allow for putting the air filters back on but now I'd be happy with any reasonable idle.

Twice I had it good enough for me but had a slight hesitation at 6,000 revs so thought I'd sort that which was silly as I don't normally go above 5-5,500 - and then Guy reminded that I don't actually get WOT on the butterflies (but that's another story). I had the bone in my mouth (twice) and then I saw the reflection in the water. :)

My settings should test out Peter's new dyno software, should see some interest shapes and patterns.
Nigel Atkins

Mike,
cracking comment.

Guy,
only just noticed the last bit on yours!

TBH I'm just trying to get to 100 posts now. :)
Nigel Atkins

I can help you with that Nigel.
Malcolm

And you just have. :)
Nigel Atkins

I'm
Malcolm

Glad ;-)
Malcolm

This thread was discussed between 12/09/2020 and 16/09/2020

MG Midget and Sprite Technical index

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