I have a generator with carburetor. It seems to consume a bit more fuel than I expected, 0.44 liters per hour for 386 watt production, or 1.14 liters per kWh. I'm also bit more annoyed by the exhaust fumes than I expected to be for a 4-stroke engine.

One factor that might affect the fuel consumption and exhaust fumes is the type of gasoline used. I understand that today nearly every location on this planet is using E10 gasoline (10% ethanol, 90% fossil fuel). However, I'm not using that and I'm not using E0 either; I'm using "small engine gasoline" that consists of only simple chain hydrocarbons and contains no aromatic hydrocarbons. The small engine gasoline has been specifically created to stay useful for 3-5 years in storage, to never block carburetors even if aged, and to evaporate completely without leaving any residue.

The small engine gasoline has bit different properties that normal gasoline:

  • 95 E0 without oxygen: 32.7 MJ/l, 755 kg/m3, H/C 1.78
  • 95 E10: 31.1 MJ/l, 743 kg/m3, H/C 1.89; 3.2% O
  • small engine gasoline: 30.8 MJ/l, 685 kg/m3, H/C 2.17

I'm trying to understand how a carburetor works. If I have a carburetor and one cubic meter of air passes through it, it picks up some amount of fuel. I understand that the fuel is in the liquid state, so the carburetor isn't pulling evaporated gas but rather liquid that forms droplets and evaporates in the cylinder.

But how is the amount of fuel picked up by the carburetor determined?

Does it pick a certain volume of liquid fuel, such as always one liter of fuel per one cubic meter of air (just an example, not to suggest that would be correct air/fuel ratio)?

Or does it pick a certain mass of liquid fuel, such as always one kilogram of fuel per one cubic meter of air?

Or does it pick a certain number of fuel molecules, such as 4*1024 molecules per one cubic meter of air?

Of course the air pressure affects the operation of the carburetor, so let's assume the altitude is fixed, and the position of the choke valve is fixed.

I quickly calculated that one liter of both 95 E10 and small engine gasoline require almost the same number of external oxygen atoms to burn (only 0.5% difference), but that one kilogram of small engine gasoline requires 8% more oxygen atoms to burn than one kilogram of 95 E10. So if the carburetor is "picking kilograms" rather than "picking liters", then its adjustment might be off for the small engine gasoline, which might explain the annoying exhaust and little bit higher fuel consumption than what I expected (however, the little bit higher fuel consumption can also be explained by 6% lower energy density of small engine gasoline when compared with oxygenless 95 E0 -- it's possible the manufacturer has stated the fuel consumption with oxygenless 95 E0).

  • A note: on the day I used the generator, air pressure was 1021.5 mbar. At this altitude, air pressure can be as high as 1048 mbar due to weather. I understand that lower pressure means richer mixture (that's what the choke valve does), so it's possible the generator has been adjusted to deliver maximum power even with 1048 mbar pressure, meaning I get 2.5% too rich mixture at 1021.5 mbar pressure.
    – juhist
    May 27 at 15:52

3 Answers 3


It's not constant mass or constant volume, but more complicated - for an ideal free jet, mass flow is area * square root of (2 * density * pressure difference) - so a higher density increases the mass flow and decreases the volumetric flow, both by a ratio of square root (new density / old density)

And if the pressure loss in the jet internal passages is significant, the viscosity of the fuel could change things as well

So really adjusting based on maths can't help you much here, just tweak things until it's not obviously rich or lean


Carburetors basically work by aspiration, as air flows through a venturi, it draws an amount of gasoline depending on velocity of the air. The devil is in the details. There will be a main orfice that governs flow of gasoline to the venturi. Also the float level will determine the liquid head pressure of the gasoline. There will be a separate idle system for low speed ( low air velocity). Automobile carbs likely have a system that adjusts gasoline flow depending on intake manifold vacuum. I have tried to modify without success; I think the best you can do it make sure it is operating as designed, no restricted passageways. There are a lot of details so the devil has a lot to do.


Ideally a carburetor would pick up enough fuel by mass so that you end up with a near desired stoichiometric ratio of air to gas. E10 has a stoichiometric ratio of about 14.1:1. 14.1 kilograms of air to 1 kilogram of fuel. Note that from the mass of the fuel or air, you can compute the number of molecules.


I liked this discussion including a short table with stoichiometric air fuel ratios:


enter image description here

Carburetors, especially small engine carburetors, are not that accurate. Plus small engines tend to have more incomplete combustion just because of their simple design.

One of the problems with using volume measurements is the volumetric density of fuel changes with temperature. On a hot day, one liter of gasoline will contain fewer molecules, fewer moles, of gasoline than on a cold day. This is why modern jet aircraft will display number of pounds of fuel, not gallons.


The amount of air that an engine is pulling in depends on temperature and pressure. Generally, on colder days with higher pressure you can make more power because you can get more molecules of air into the engine. Boyles law or the Ideal Gas Law is an explanation:


PV = nRT

Pressure * Volume = number_of_moles * Reynolds_number * temperature

At wide open throttle, a small engine should be able to pull in 75% or more of its displacement volume every two cycles for a four stroke.


With the atomospheric pressure and the temperature you can estimate the number of moles of air that the engine gets into the combustion chamber.

From your description, the engine sounds like its running rich, high fuel consumption and smoke. My first reaction would be to put in a clean air filter, plug and replace the main jet on the carburetor, possibly jetting down one or two sizes, if it is replaceable. In my motocross years, I found that the labels on the jets, the amount of fuel they should flow, can be off multiple sizes. I would find this by timing the amount of fuel that the jet could flow through a burette. Some were good, some were not. Also, if you are at a higher altitude or hotter temperature, you will need to jet down.

It would be interesting to see a data sheet for your "small engine gasoline".

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.