How do you open the carburetor secondaries?

[PEntertainment]

What are the secondaries and how do you open them up? By flooring it? Is that something to do with the carb? I have a 454 in my elandan - Novice question I know -

[RockwoodMike]

Take a look at this picture..The smaller round openings are the primary ports (openings to let air into the engine) and the bigger ones are the secondary ports or simply "secondaries"..These secondary ports open when you really need a lot of air (and fuel mixed with it) to have lots of power from your engine..

But through the design of the carburetor ,these secondary ports do not open till just about all the air that can go through the primaries is going through them..Then the secondaries will open up..and it is done(the opening of the secondaries) by a thing called venturi vacuum..As a lot of air is rushing through the primaries, there is a small opening in the primaries where air can be sucked in..this creates a vacuum at this opening (it's called ported vacuum) and this vacuum is used to open the secondaries (by way of a diaphragm and mechanical linkage).

So at a stop light, your engine is idling ...the carb is only letting in a small amount of air and fuel through the primaries..because the secondaries are completely closed and the primaries are barely open a small amount..That is how the idle is set..open a touch more and your engine idles a little faster because it is sucking in more air and fuel..close it a bit and less fuel and air so it runs slower

It would be like if you had two noses on your face..the primary nose lets your lungs suck in air while you are just sitting in a chair or a casual walk..That primary nose can handle all the air you need for this..but start running up a steep hill and your secondary nose gets a message to open up because the primary nose is maxed out in regards to letting air in..the secondary nose opens then and you can get more air into your lungs ( I know this is a terrible example...sorry)...

So when you stomp the throttle..the secondaries are not going to open now..not until the primaries have just about done all they can first..then they signal the secondaries to start opening..This opening of the secondaries take a bit of time to open..

your idling at 700 hundred rpms..stomp the throttle..it might take several seconds for your engine to get to 2700 rpms (just an example #) especially as it shifts the gears..slowing down the rpms..your primaries can handle all the air your engine needs until 2700..then the primaries are maxed out and the secondaries are signaled to open..to go to 4000 rpms..

[PEntertainment]

Ahh I get it. I thought it was something like that. In this other post they were talking about the secondaries opening up when going up hills. When I was cleaning my carb I saw 2 more ports open up as the RPM went way up. I could see the gas flowing a lot more. 2 noses - I like that analogy haha. Thanks for the detailed info!

[doylexl]

Internal combustion engines are basically air pumps that produce power that move things like cars and motorhomes.

When an engine is running air is going out the exhaust and in thru the intake/carburetor. The carburetor's purpose is to limit the amount of fuel and air going into an engine and when an engine is running it is pulling the air in thru the carburetor which is a suction sort of like a vacuum cleaner and one way to tell if your engine is in good condition is the amount of vacuum at the intake.

Carburation worked for many years but as engines wear and carbon builds up the carburetor is still set or adjusted for a new engine and thats why fuel injection is much better because a fuel injection system is constantly watching the sensor inputs to the computer and adjusting the air fuel mixture for the optimal mixture which is called stoichiometry which is the perfect fuel to air mixture which is 14.7 to 1 air to fuel. A large engine in a motorhome will typically have a four barrel carburetor and most of the time the engine is only using the two small ports which are called primary and when you need considerably more power and push the accelerator farther down the secondary ports will open so that the engine/air pump can breath even deeper and turn more rpm and make more power to carry you up the hill or pull the heavy load.

Diesel engines don't usually have much vacuum because they have 4 times the compression as a gas engine so diesel engines normally have a vacuum pump to make vacuum for things like power brakes and ac vents . Four barrel carbs either have mechanical secondaries or vacuum operated secondaries. The mechanical secondaries will open every time that you push the accelerator pedal far enough but vacuum secondaries will only open up if the engine is making enough vacuum to open the secondaries which is why sometimes vacuum secondaries will get stuck due to rarely being pushed hard enough to open.

A two barrel can have primary and secondary ports and they some times only have primary ports and they always open together.

[RockwoodMike]

Another fun fact is the 14.7 air fuel mixture ratio..That ratio is in weight..not volume..

Doing some Google fact checking.. A gallon of gas weighs about 6 pounds..A pound of air...at sea level..70 degrees temp...so many variables like humidity and stuff but the general number for a pound of air is 13.4 cubic feet..

13.4 cubic feet times 6 equals 80.4 cubic feet of air...So 80.4 cubic feet of air weighs the same as a gallon of gas..

But the ratio is 14.7/1...14.7 of air...1 of gas...in weight!!

So 80.4 times14.7 equals 1181.88 cubic feet of air.

For every gallon of fuel that the carb mixes with the air, it must use 1181 cubic feet of air..

Carbs are kind of iffie on doing that..when your idling or taking it easy on the throttle..it does a pretty good job..

But if you could watch the top of the carb as it is wide open, under heavy hill climbing load...it is just pouring the gas in.   not very efficient..

Also with that volume of air, keeping the air cleaner maintained is a good idea..not only to keep the dirt in the air from being sucked into the engine

but if the air filter is restricted, it can't do a good job of trying to make that 14.7 ratio..