There's a question that is pretty similar to this one except the existing question satisfies EFI engines.

EFI question

Let's say I roll down a hill at a considerable speed I let go of the gas. Say my revs are 3000.

While going down the hill my revs will remain at 3000 but I'm confused on whether or not the carburetor keeps on sending gasoline to the engine cylinders for combustion.

My uninformed guess is that it sends a minimum to avoid the car from slowing down and losing momentum.

3 Answers 3


At a base level, carburetors meter the amount of fuel they let into the engine by the amount of air that is moving through them.

Vacuum is created by the piston moving in the engine and creating an open space. As the piston moves down, it creates an empty volume which pulls in air through the only opening it can find, which is the passageway through the carburetor. So, for example, a 25 cubic inch cylinder is going to try to pull in 25 cubic inches of air in through the carburetor.

However, if the throttle plate is partially or fully closed, it's not going to be able to get that much air in since there's a restriction (much like trying to take a full breath of air through a straw). The more the restriction, the higher the vacuum.

Generally, the more throttle you give it, the more the throttle plates open, and the more air it can get in, so the less vacuum there is. Designers quickly realized that the higher the engine vacuum, the less fuel should go into the engine (since more vacuum means less throttle is being applied). And visa versa the lower the vacuum, the more fuel should go in. From there, they set up all sorts of metering tools in the carburetor to do that.

So, that being said, when you let off the gas and coast downhill, the throttle plates are fully closed and your engine has a LOT of vacuum, especially at high RPMs. Since that's the case, even though the engine is spinning very fast, there's very little air moving through the carburetor since the plates are closed. In this condition, the carburetor will go into idle mode and release as little fuel into the engine as possible.

The only issue with this is that when you're going downhill in the condition you described, there is a lot more vacuum than there normally would be at idle due to the high RPMs. At idle, a healthy engine creates around 18 to 20hg of vacuum. When you're coasting downhill with the throttle closed, it can get as high as 25hg or more. So, even though the carburetor isn't designed to release a lot of fuel into the air stream, the extreme vacuum can actually pull fuel out of places it's not supposed to normally come from under those conditions, which can make the engine run rich.

Either way, the carburetor doesn't care if your engine dies or loses momentum. It just meters fuel into the air passing through it. That's all it does and nothing more. If you're coasting down a hill at 3000 RPMs, your carburetor only meters fuel into the engine because air is passing through it. The only thing keeping your engine speed or momentum up is the spinning of the tires, the momentum of the vehicle, and gravity.

  • Welcome to Mechanics.SE! Awesome first answer! Commented Apr 14, 2016 at 22:51
  • Hey, nice answer. +1 Commented Apr 14, 2016 at 22:58
  • Interestingly, on some older, larger engines, the amount of fuel metered into the engine under these kinds of circumstances is not sufficient for the spark plug to ignite, and it exits the cylinder unburnt. It then usually ignites somewhere in the exhaust system, causing rather amusing popping noises, especially when going down steep hills under heavy load.
    – Perkins
    Commented Apr 15, 2016 at 0:16
  • @Perkins That's interesting. I'll test mine down a steep hill
    – EChan42
    Commented Apr 22, 2016 at 10:06
  • @EChan42 Of the vehicles I've driven that exhibit the behavior, the newest one was a '67 Chevy, and all of them have been rated for loads >= 1 Ton (so larger engines) Don't expect it on anything that's computer-controlled, those usually do too good a job of turning the fuel off.
    – Perkins
    Commented Apr 25, 2016 at 19:17

Carburettors are very crude in comparison to EFI systems, and so the amount of fuel entering the engine is simply a factor of the amount of air going in, which is controlled by the position of the butterfly (and hence by the throttle position).

At a completely closed throttle, there will therefore still be some fuel getting in, enough to keep the engine running without any load, as the system has no way of knowing that the car is going at speed.

Controlling the air/fuel mixture in a carb is a real 'dark art', and the experts can spend huge amount of time tinkering with tiny adjustments, often filing tiny amounts from the needles to tweak the mixture at a certain range...


Carburetor Circuits Will Still Pull Fuel from the System

If the engine is running on a carbureted vehicle, off throttle or not, it will consume fuel.

Throttle Settings

There are three basic circuits in a most carburetors that provide fuel to the ICE.

  • Idle Circuit - effects fuel metering at low RPM conditions where the throttle plate is closed.

  • Secondary or Mid-range Circuit - effects fuel from off throttle (throttle plate closed) to wide open.

  • Primary or Main Circuit - Feeds and rises in fuel delivery output as the throttle transitions from partially open to wide open.

Other circuits include accelerator pump circuits as well as choke circuits that aren't pertinent to this situation.


When you are going downhill and in gear your engine pulling air threw it. Which is it's purpose. It's simply a big air pump. Since air is flowing into the system and it is getting pulled through the carburetor with a closed throttle it will be receiving fuel into the system through the idle circuit.

It will not be consuming the same amount of fuel per RPM that it would receive if the the throttle was wide open but it will still receive fuel, unlike an EFI system that could cut out fuel completely.

Simple Answer

Your engine will still consume fuel under the conditions you alliterated.

It is not providing fuel to maintain your downhill speed.

It is not providing fuel for the vehicle to maintain momentum.

It IS providing fuel because it cannot do anything other than provide fuel at the throttle position that it is at, in this case it's the idle circuit. So deceleration is done at the idle circuit, right?

When you have popping on deceleration it's usually the idle circuit. I've fixed a lot of gas high speed backfiring and i've always adjusted the air/fuel screw on the idle circuit to get rid of it.

My 2 cents.

  • 1
    Indeed! Keep coming back. You had a great well worded question. We need your questions, it's our life blood here. So thank you too. Commented Apr 15, 2016 at 8:17
  • 1
    My life blood is my little Citroën BX so as long as you guys provide me an insight to understanding it better I'll be happy to be a familiar face around this place :) Thank you for your kindness!
    – EChan42
    Commented Apr 15, 2016 at 8:20

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