In most modern engines the intake air parameters such as temperature, pressure, humidity etc can be monitored via sensors and so the exact fuel mixture can be trimmed to best efficiency.

Still aren't air parameters affecting the engine operation thus it's consumption as well? If not in modern engines, what happens in older atmospheric carb motors???

  • If you got really unlucky, with a cold engine you got icing inside the carb which locked up the butterfly valve attached to the gas pedal, at which point taking your foot off the gas had no effect. Putting the car out of gear to slow down usually stopped the engine by over-revving it! The fix was just to wait a few minutes till the ice melted, and hope by that time everything was warm enough so it wouldn't happen again.
    – alephzero
    Dec 5, 2018 at 9:51

2 Answers 2


The approach that manufacturers took differed greatly. In the most simple systems, you as the driver had a manual choke which adjusted the position of a choke flap upstream of the fuel jets. Some airboxes were designed so that in winter, the inlet tract could be rotated through 180 degrees to induct warm air from near the exhaust manifold as opposed to ambient air in the summer. Some very early vehicles even gave the driver advance and retard controls on the steering wheel to adjust the ignition advance.

The approach that more modern carburettor systems took was diverse to say the least and on certain carburettors, you'll see multiple approaches. One of note is Austin Rover which created a system which employed an oxygen sensor which fed back to electronics which controlled the carburettors fuel mixture screw using a motor on a worm-drive screw. It's worth adding that this system didn't work particularly well.

The carburettor I have on my bench at the moment is a Pierburg 2E2 which employs a fairly complex system. There is a "waxstat" which is quite literally a pin attached to a plug of wax which draws a feed from the cooling system. When cold, the wax is hard so the pin protrudes. As the system warms, the wax melts and the pin retracts adjusting the idle speed.

The choke flap is controlled by a combination of a bi-metallic spring which attaches to the spindle and again takes a feed from the engine coolant. As the car warms, the bi-metallic strip opens the choke flap. The choke is two stage in it's operation, returning to a stop based on the operation of the pull-down unit, an adjustable diaphragm with a vacuum feed, plus a set-screw for off-throttle. There is also a three-point (or four-point) unit which sets idle and fast-idle speed once the system has warmed up and come off choke. The four-point version includes a temporary enrichment for when you come from trailing throttle back to idle, intended for automatic transmission, which operates below 18 degrees celcius. Again, this uses a vacuum diaphragm, a loop of vacuum pipe and a restrictor in the incoming vacuum.

Other things you can set on various carburettors as the mechanic setting up a car are things like float height which governs how much fuel in held in the float chamber. You'll encounter things such as air drillings, emulsion tubes, main and idle jets and venturis. All of which are selected to provide specific amounts of fuel, air and how it all mixes. You'll also find inlet manifolds heated by coolant, by electric heaters and by exhaust gas.

On systems such as the SU carbs that classic Minis use feature a needle which sets fuel mixture by going into and out of a drilling. The needle can be changed, the strength of the spring can be changed and the top of the needle sits in an oil-filled reservoir.

Whilst I realise this isn't a shopping advice site, I can whole heartedly recommend you take a look at the Haynes Carburettor Manual ISBN 1859602886 if you want to read more on the subject.

  • Argh.. manual chokes! I've been trying very hard to forget about those :D Dec 4, 2018 at 11:43
  • 1
    Then there was the Reece-Fish carburettor.... can't remember if the names are correct...
    – Solar Mike
    Dec 4, 2018 at 12:25
  • Oh yes, Reece-Fish. I had friends with these but never got my hands on one myself. There was DellOrto which had fantastic atomisation and mid-throttle response, Weber with the ubiquitous DCOE, Solex (which was essentially Pierburg) plus some Japanese ones, Mikumi I think they were called? Pleased someone asked a question in carbs as I’m more more confident with them than injection! Dec 4, 2018 at 17:25

Yep they do effect the fuel mixture and thus performance of the engine.

The adaption of the fueling to maintain the correct Air Fuel Ratio (AFR) is less about "best efficiency" in a fuel consumption sense and more about keeping the engine healthy. Too much air (running lean) and you'll get too high temperatures in the cylinders and pistons and you run the risk of the car seizing, and of course the exhaust gas temperatures (EGTs) go up as well which can damage, turbos etc. Too little air (running rich) and you'll get poor consumption, you'll be reducing the lubrication provided by the oil and the excess petrol gets expelled through the exhaust killing O2 sensors and catalytic convertors given enough time.

Modern engines use the various sensors - Mass Air Flow (MAF) being the primary one for this, to adjust the amount of fuel injected to match the amount of air available. Where the environmental conditions give you "less" air (altitude, high temperature etc) you get less fuel - the engine will be producing less power so you'll need to give it more throttle to maintain the same speed (so your consumption goes up) but the engine will be "safe", when you have "more" air (low altitude, cold temperatures etc) the ECU will inject more fuel and more power. Since the ECU will strive to maintain the correct AFR regardless of whether you are using the extra power provided by the larger amounts of fuel and air you pay an increased consumption penalty regardless (one of the reasons why you're m.p.g drops in winter!)

what happens in older atmospheric carb motors???

In much the same way - but the car couldn't adapt itself. This is why everyone hates a carb'd car on a cold winter morning! Often the carb would need to be adjusted seasonally.

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