Carburettors!

OK chaps, in words of one syllable for my benefit...

Carburettors, how is it they can differ??

OK I understand that a carb mixes fuel and air to send to the cylinder. It does so in a narrow part of the carb which ups the air pressure or summat so that you can squirt in the fuel through a jet to atomise it.

Sooooo

1. Why do they have more than one jet? And what difference does different sizes of jet make?

2. What difference do different size carburettors make?

3. What difference does the length of the needle make?

And anything else anyone may care to add. Not much then

Cheers

Edited By Wolfie on 29/04/2016 18:50:04

its simple really ,as the piston goes down ,it creates a low pressure in the cylinder the inlet valve opens and air rushes into the low pressure area through the carburetor as it passes over the jet it sucks petrol out of the jet into the cylinder. the bigger the jet the more fuel the bigger the carb more air. theneedle is tapered and controls the amount of fuel. small amounts for slow running larger amount for fast running, some carbs do not use a needle and have more than 1 jet to achive a similar result.

There has been an excellent series in ME about infernal confusion engines, it had a good bit about carbs. Buy yourself a digital subscription and read it up.

AIr flow and pressure is all about the Bernouilli Principle. Higher the air flow, the lower the pressure. Think here of blowing between two pieces of paper, when they actually move closer together rather than further apart. Or aeroplane wings creating lift.

The carburettor has a choke tube, or venturi, where the air is speeded up as it passes through. The lower pressure 'allows' fuel to be drawn into the air stream due to the dfference in pressure c/f the fuel supply (which is maintained at a constant level wrt the jet). Different sized engines need different sized venturi and different sized jets. Some, like the ubiquitous Webber 28-36, had two venturi for differing engine speeds and most need some form of help to maintain adequate operation at engine idle, hence a slow running jet and a power jet(s). Other improvements, to aid atomisation and better mixing of the fuel and air can make carbs quite ccomplicated.

Some carbs have simple fixed jets and some have jets with a 'needle' as control. That can be a fixed (but adjustable) or movable item depending on the type of carb.

Hope that helps a bit.

RAB

Edited By Richard Balderson on 29/04/2016 19:59:16

1. Some carburettors only have one jet, big jets for big injins.

2. Big carburettor for big engines.

3 Some carburettors dont have needles.

4. Some old carburettors was a tank that bubbled air through it on the way to the engine bit.

And to complicate matters there is fuel injection all done without a needle.

Edited By Dod on 29/04/2016 20:30:25

And just to top it off Fish carbs dont have needles or jets.......b

This a complicated subject Wolfie, Firstly consider how the engine works, you have an idling condition where fuel is sucked in at abut a 16 to 1 ratio, air to petrol. The throttle stop is set to get an idling speed and the fuel jet adjuster then set to give a good transition to the throttle opening condition where the engine starts to speed up. This where the accelerator pump comes in as it gives squirt of fuel so that the engine does not stall. By that time the fuel and air flow is adequate to give the engine power and revs. With needles they are tapered so that more fuel comes through as the taper is exposed from the jet.

This a very simplistic explanation and there is a lot more sophistication involved. Things like fuel injection and turbos.

With Engine Control Management, the whole thing is electronically controlled where the engine timing is adjusted while running and the air intake and humidity are measured as well.

The days of home adjustment are long gone, unless you own an older car.

Clive

The carburretor has been described as a device to deliver an incorrect mixture at all engine speeds.

Fixed Jet

With a single jet, and fixed venturi, the fuel passing through the jet will only provide the correct air/fuel ratio at one engine speed. To expand this range, more sophisticated units effectively put the Main Jet out of circuit at small throttle openings, and take fuel through a smaller, Idling Jet. Again, this is not ideal, so arrangements are made for air to bleed into the fuel stream, to weaken it as the speed rises. The extra air to create the air/fuel emulsion is controlled by the Idling mixture Screw, (A needle valve actually). The idling system, usually delivers air beneath the throttle plate.

As the throttle opens and the air flow through the venturi increase, the main jet comes into play, but the air/fuel delivered is modified by the Compensating Jet, which can admit extra fuel to improve mixture strength at relatively low air flows. As the air flow increases, and the fuel delivery from the Main Jet increases the Compensating system decreases the fuel flow and can admit extra air to prevent the mixture becoming over rich.

Constant Vacuum/ Variable Venturi

The best known exponent is the S U. This uses the depression caused by the air flow through the variable venturi to lift a dual diameter piston which carries a tapered needle, (which is centred in a fixed fuel jet). As the air flow increases, the depression increases, because of a greater throttle opening, and or higher engine speed, so the piston rises, increasing the area of the venturi, and decreasing the depression acting on the jet. To compensate for this, the tapered needle , being higher, increases the area through which fuel can flow. By carefully matching the profile of the needle to the air flow of that particular engine configuration, the air/fuel mixture delivered to the engine can be fairly closely controlled. This has the advantage of providing minimal restriction to air flow, and therefore volumetric efficiency at high engine speeds and large throttle openings.

At lower speeds, the smaller area venturi improves fuel atomisation, and hence mixture distribution, so giving better low speed torque.

The Stromberg version of this, replaces the larger diameter of the piston by a "rubber" diaphragm, which has the advantage of providing an air tight seal; but the disadvantage of having a spring rate, so that the resistance to lifting increases as the diaphragm and smaller piston lift.

Ford also produced a variation on this theme, using a pivoted sector to move the needle in and out of the jet.

In every case, the profile of the needle has to be matched to the particular engine and its air flow

Twin choke, progressive, carburettors are an attempt for fixed jet units to match the characteristics of the constant vacuum units. Some use a mechanical linkage from the primary throttle to open the secondary throttle more rapidly as the primary reaches a certain opening. Other versions, use a vacuum diaphragm to open the secondary throttle.

The opening points need to be carefully set to ensure as smooth a changeover as possible and to optimise performance..

So taking a carb, off, say, a 3.4 litre Jaguar and bolting it straight onto a 1.8 Litre BMC B series will not provide optimum performance or economy. With a suitably matched needle, the performance can be improved..

In the same way, fitting a pair of big bore Twin Choke Webers on to a 850 Reliant engine will improve breathing, but the jets will have to changed to match the air flow characteristics of the smaller engine.

Changing the camshaft to one giving different valve lift, or events, will call for ta change of jets.

A carburettor which is badly matched to the engine will not provide smooth driveability, torque, power, or economy. And, it may make the engine an absolute pig to cold start!

Electronic Engine management, controlling fuel delivery, from the Injectors, and Ignition Timing, gives much better control since the fuel delivery can be adjusted to the required level by feedback signals from the Lambda sensor, which should optimise all engine settings, for emissions and power output.

Howard

Wolfie, those three innocent looking questions can have very complicated answers.

To keep it simple, and hopefully not simplistic, an engine needs a supply of a mix of c.16 parts air to 1 part petrol for normal running. There is normally a large hole through the centre of the carb where this mixture is created. The air comes from outside through an airfilter and the petrol is fed in through small holes called jets. These jets can vary in size of the orifice and from small to large of course supply varying amounts of petrol.

A simple carb has the jets or orifices calculated in the factory and they cannot be varied. eg many cheap lawnmowers etc. They are calculated to work well over a fixed speed range.

A step up from that on machines which need to operate on a wider speed range will have a basic setup to control the mixture at three critical points ie. at starting from cold, then at tick-over (or idling speed) and lastly at full open throttle.      At tick-over a small Idling jet feeds out a small amount of petrol into the airstream. This can usually be adjusted by means of a tapered pointed screw, the nose of which can be screwed into the little hole or jet to increase or decrease its size and consequently the flow of petrol that it delivers. Generally called the idle/ mixture screw, you set it when the engine has warmed up to normal operating temperature. Once set you usually leave it alone.

So until the engine warms up another scenario prevails. A cold engine starts better on a rich mixture, ie. more petrol in the air/petrol mix. To achieve this there is often a flap of some sort called the Choke which restricts the flow of air so the mix going into the engine drops to maybe 10 to 1 for starting purposes. As the engine gets going and warms up you would reduce the choke amount and after a few minutes the engine will run on the idle jet as above.

The third state of running would see the engine at full speed and running on what would usually be called the Main jet. This is usually the one you find sticking up slightly into the mail hole through the carb. To vary the amount of fuel drawn through the main jet there is often a tapered Needle that is set with its nose partly inserted in the main jet orifice. The more this needle is inserted into the main jet, the smaller the opening and vice versa. This needle is controlled by the accelerator cable and can be raised or lowered by the operator to increase or decrease the flow through the Main jet and thereby regulate the speed of the engine.

To get quickly from running on idling speed to running full speed there is often as Clive states, an accelerator pump that gives a little squirt of fuel that helps things along.

There are many refinements and adjustments on the above. A good source for more detailed explanation is:

**LINK**

John

Edited By Breva on 30/04/2016 00:12:37

Edited By Breva on 30/04/2016 00:16:56

If you are talking about SU or Strombergs try reading Vizzard's 'tuning the A series engine'.
(He doesn't only cover bmc A's and B's)

Much testing on a gas flow bench showed a single 1 3/4" SU would flow more mixture than the more widely accepted twin 1 1/4" SU setup. (assuming the manifold wasn't strangling it).

He has several chapters on SU jets/needles/springs.

Lots of insights that go against received wisdom. For instance; traditionally, siamesed inlet ports had the web sharpened to a point where you could cut your fingers on them.
According to Vizzard's data, a blunt bullet shaped section flowed better because much of the time the mixture was flowing across the web rather than to one side or the other.

Ports were polished to mirrored perfection.
Again his data showed a slight roughness actually improved flow and helped prevent the mixture condensing on the walls.

john

Thankyou lads some excellent info there

I had one hell of a job getting the three SU carbs fitted to the pre-war BMW engine in this beast to come onto song!

Russell

I believe the term carburettor comes from French, and refers to bubbling air through the fuel, creating a vapour strong mixture, see the motors by Jan Ridders.

Ian S C

I found that the SU carburettor was an excellent simple device which could be easily adjusted to suit the engine concerned. (No, I'm not talking about belching black exhaust either, just the correct state of tune for that particular engine.) I also found that the BMC A series and later versions (R series? ie whatever the Maxis had) were excellent at starting at all times whereas other vehicles appeared to struggle to start under adverse conditions. I put this down to the fact that the SU cold starting device (aka choke) both opened the throttle slightly, and moved the jet down thus allowing a richer mixture to flow. As far as I know, other carburettors of the time, didn't enrich the mixture.

Bearing in mind that I drove a Minivan or a Minor van throughout the 1962/63 winter and these vans were parked outside, and there was quite often two or three inches of overnight snow on the vans, the procedure I adopted was to unlock the van, lean in, waggle the gear lever to ensure neutral, insert the key, pull out the choke, and operate the starter for a two or three seconds, and only then start to clean the snow off the vehicle. Often, the engines would start after I had released the starter.

Another advantage of these carburettors was that the first half-inch of the choke simply operated the throttle without affecting the jet position: this came in very useful when the accelerator cable on one of my Maxis broke as I was able to use the choke control as a (admittedly limited) hand accelerator until I could obtain a new cable.

Happy days.

Peter G. Shaw

On most fixed jet carburrettors, as on constant vacuum units, the initial throttle opening on full choke was vital to ensure a start. Presumably, this controlled the air flow, and probably, the fuel spread over the throttle plate to give increased area for vaporisation. As an instance, the Solex fitted to the 1198 Ford Anglia engine called for a the throttle to be opened, on full choke, to just allow a No 57 drill between the throttle and the carb body.

Getting three identical carbs on song on an in line six cylinder engine may well be difficult, since each one is not subjected to equally spaced induction pulses. In all probablility, they will each need to be set up differently, because of the variation in interval between pulses. ( 1 and 2 are separated by three cylinders, 3 and 4 by two, and 5 and 6 are consecutive for Firing order, and therefore induction pulses for 153624. For the reverse Firing order of 162453; the pulses for 1 and 2, and 6 and 3 are separated by one cylinder, but 4 and 5 are consecutive.

Two carbs on a six, each feeding one end of the engine will provide equal pulse spacing. Which is why, often, the exhaust offtakes are from 123, and then from 456.

For an in line four, ideally, 1 and 3 would be fed from one carb, and 2 and 4 from another. Since this is difficult to arrange, a balance pipe allows some of the induction pulse to be felt by the carburettor at the other end of the engine. But, a balance pipe with too large an area can make acceleration less sharp, since it begins to approach the effect of two carburrettors feeding a common inlet manifold, and reducing the depression on each one.

The object of having equally spaced pulses is to have uniform air/fuel ratios for each cylinder, to maximise the torque under any speed/load condition.

Vee or boxer engines are an entirely different matter! Just look at the complicated inlet manifold passages for a V8 fed by a four barrel carb, in the attempt to produce equally spaced pulses!

Timed injection into the ports, afforded by modern electronic control systems, does away with such problems.

Howard

I'm confused. A constant vacuum carb usually has a piston that rises and falls, so the tapered needle forms a variable jet.

And the last time I looked, cylinders 1 & 4 rose and fell together while 2 & 3 rose and fell together. Assuming we are talking about 4 strokes and an I-4 layout, you want to share 1 & 4 on one carb and 2 & 3 on the other.

In fact, one of the problems with the A Series is that inlet ports 1 & 2 are cojoined, as are 3 & 4. The induction pulse are thus far from ideal and it's very difficult to get even sharing of the mixture.

Is it me?

Edited By Muzzer on 03/05/2016 16:52:35

David Vizard (pronouced Vyzard) is still going strong although he moved over to the States some years ago. He clearly focusses very much on pushrod engines, particularly porting and bench flow, cam timing and blueprinting. He's done a series of seminars that you can find easily on Youtube and only a couple of years back looked as if he was going to become regular contributor to PPC Magazine, the spiritual successor to CCC Magazine. The latter role didn't seem to work out for some reason (wrong side of the pond?) - pity.

His first book "How to Modify Your Mini" (1977) remains almost imprinted on my mind, although I moved on from the quaint BLMC stuff to Japanese engines once the penny dropped. I once bumped into Vizard at Piper Cams in 1981 or 1982 when I was at Ford Dunton - Piper FM did (still do?) contract engine research programs for them - fuel injection, flow studies, stratified charge etc. Good to see him still thriving although seems to me he is sort of stuck in the past, hence his focus on US pushrod engines I suppose. He also seems to have a FB page.

Murray

SU on eth Marina was always well behaved, even if the rest of the car was dire.

The Ford VV was a nightmare, especially the auto-choke. I fitted a manual choke (2L Cortina estate) which helped until a linkage dropped into nowhere somewhere north of Ullapool... fixed with a paperclip! replaced with a twin-choke Weber off a transit, which gave me incredible low-down grunt (especially with the low-geared estate diff). Fuel consumption was appalling and the diff went bad after a year or two.

Don't remember what was on the Manta (it was the non-FI version) but it gave no trouble, subsequently all the cars have been fuel injection 'which just works' (hopefully).

Neil

Aha, that funny Ford variable venturi thing? That was Ford's means of getting around the existing patents held by the likes of Stromberg etc. And it was at the end of the carburettor era, when purely mechanical means were struggling to keep up with the emissions legislation. Amazing that carburettors managed to cut the mustard for so long, given the massive range of operating conditions they had to cope with.

Around this time they were fitting the K Jetronic (purely mechanical but very consistent) and L Jetronic (proper ECU, fully electronic) injection systems from Bosch - but only to the higher end vehicles due to cost. Almost all diesels fitted to Fords then were from PSA, with mechanical injection.

Murray

Muzzer,

I'm no expert so could be completely wrong here, but I always thought that the SU carburettor, as used on the A series engine was variable vacuum since the position of the diaphragm, or whatever it's called, was dependant on the throttle opening - closed = high vacuum, whilst wide open = low vacuum. This is dependant, is it not, on how much suction, hence vacuum, the engine can produce which means that if the throttle is wide open then the engine can't produce much suction since there is no restriction on the air flow.

Think of the old vacuum operated windscreen wipers which would go like the clappers when going downhill, or when changing gear, but almost, if not completely, stop, when pulling hard, eg when going uphill. Hence variable vacuum.

Neil and others,

I always found it very interesting that my parents used to talk about "catching it" when attempting to start their earlier Fords, and if they didn't get it right, it wouldn't start, whereas at work, where Morris ruled the roost, there was never any problem - a quick flick of the starter and away they went.

Peter G. Shaw