Carburetors

[Evo7GTA]

Hey guys, I was just wondering if you could give me information about carburetors.
- What are the different types of carbs
- How do they work
- Tunability
etc.

I know it might seem like alot but leave nothing out, I'm a fast learner when it comes to cars and I'll read every bit of info you give me.

Thanks

 

[nightkid86]

www.howstuffworks.com

 

[skip0110]

A carburetor atomizes fuel as it is drawn into the engine. The air speeds up as it passes through the neck of the carburetor (barrel) and causes a pressure drop, drawing the fuel in and atomizing it. As the throttle is opened, the throttle plates rotate allowing higher air velocity, and thus drawing more fuel into the engine. Fuel is drawn in through jets--picking the jet sizes determines the A/F ratio and is often the easiest part of tuning.

Because if the throttle plates are opened too quickly, too much fuel can be drawn in (causing flooding), the throttle plates in 2 of the 4 barrels (or narrow opening) of the carburetor are linked to engine vacuum--a greater engine vacuum means that the engine can handle a greater amount of fuel and allows these secondaries to open. Getting the secondaries to open at full throttle, yet not open too quickly and cause flooding, is the second most difficult part of tuning a carb.

There is another set of plates, called the choke, which close when it is cold out, allowing the engine to run rich and idle in colder temperatures. The choke may be manual (on very old cars), vacuum, or electric. Often tuning the choke to provide the right amount of enrichment is the hardest part of tuning a carb.

The is one more primary part of the carburetor, the accelerator pump, which sucks extra fuel into the float bowls (where the fuel is stored) under rapid throttle application so they do not dry out and cause a lean condition.


I think that about covers it.
HTH.

 

[Super Jamie]

i would actually say getting your throttle response snappy at low-mid rpm is the hardest part of tuning a carburettor, which relies on airspeed (which you change by modifying the throats) and amount of fuel being supplied at the time, which both the jets and the accelerator pump are responsible for in this instance. even some factory installs of carbs have a tendancy to "bog down" if you floor the acclerator pedal, as there is too much fuel, too much air, or both. this is from me who doesn't run a choke anyway

there are different types of carbs, mainly downdrafts which suck air from the top, add atomised fuel to it and the mixture comes out the bottom. things like holleys and dominators are downdrafts, as well as alot of webers like DGVs and IDAs. sidedrafts and SUs (skinner union) suck air in from the side and it comes out the other side, these have names like weber dcoe and dellorto. there are also updraft carburettors but these are less common and i don't know anything about them

it's generally accepted that sidedraft webers and dellortos are about as good as you can get. they have superior atomisation to almost every other carb out there, so you get a better burn and make more power for the amount of fuel you're using. on a four cylinder (or straight six) you're going to be sucking from the side of the head anyway, so a sidedraft has a less restrictive intake tract than a downdraft, less restriction is good. v8s i would expect a downdraft would be a better option, as there are millions of manifolds available for them, and their inlet ports are in the middle of the "valley"

most carbs have throttle butterflies, which open and close gradually to allow an amount of air down the venturiis (skinny part of the throats that causes pressure drop). a decent replacement for sidedrafts is SU carbs, which have a sliding venturi type design, and work partially off vacuum. most old carb experts can tune SUs perfectly by ear, however they have a tendancy to go out of tune fairly quickly

compared to efi, carbs are a compromise. they will be as efficient as efi at WOT (wide open throttle, or full airflow capacity) providing you get your fuelling right. but anything below that (circuit driving, town driving) they will not be as good. that being said, you can stay up to all hours of the night tuning carbs yourself, it costs you nothing, and it's fairly easy once you spend alot of time learning how. as opposed to having a programmable computer that you need to pay someone else to dyno tune during business hours only

alot of amateur racers run carbs for this reason. it's acutally quite amusing to turn up to a track day to see an old escort or something, with a modern sr20 or similar, with a dirty pair of carbs slung off the side even open wheelers with big-dollar rare genuine cosworth engines run carbs, so it's not like you're restricting yourself by using them

 

[slimer90210]

i would actually say getting your throttle response snappy at low-mid rpm is the hardest part of tuning a carburettor

IF you want good low end response a Quadra jet is a good carb to start with. They have smaller diameter primaries and larger diameter secondaries. If you know how to set them up (or know someone that does) you can get a carb that has very good low end response and will feed just about any sized engine around. The trick is getting a Q-jet not to bog down when the big-arsed secondaries open (I had one on a 305 and it could flood out the engine out while it was running). With the small primaries the Q-jet develops high air speed in the primaries at low engine speeds allowing for the throttle response. And the big secondaries allow for high cfm at high engine speeds. The guy that tuned my carb said the trick to a Q-jet was to make the secondaries engage only at 3000 rpm or higher and to make them open slowly until they are 1/2 open. After he tuned mine that little 305 really picked up. The low end throttle response was amazing and the bog down that happened before when the secondaries engaged was gone...

 

[Super Jamie]

except my car has a 1300cc engine, so any v8 4 barrel is going to be way overkill

but your idea is right, small primaries get your airspeed up and give good throttle response. even then, the tiny factory carburettor from my engine, on a fairly modified motor, still didn't give ideal response until i did lathe mods to the venturis and changed the jets (and got it right in an educated guess)

 

[Emohawk]

Okay, I've got a question now.
There's a guy down the street from me who's building a 240Z to compete with a guy the next block over (what a great neighbourhood, eh?). The guy a block away has 3 carbs, I think he said. The guy down the street is building his engine with 6, that's one per cylinder. WTH? Is this an advantage in any situation other then an "I have more carbs then you" match? Wouldn't it be a major pain in the ass to get them all flowing the same amount and responding the same?

 

[Super Jamie]

you're right. a single carb per cylinder (probably motorbike carbs?) would probably give better flow, but to keep it in an ideal state of tune you'd need to tinker around with it every second weekend. in my opinon, a few hp at the top end isn't worth that much effort. sidedraft webers have two throats, so having one throat per cylinder (3 carbs on a straight six) is almost the same thing, and a hell of a lot more friendly to drive and to maintain

you may have seen a guy out there, who has put 8 turbos on a v8, feeding into 8 carbs. now THAT would suck to keep in tune

 

[skip0110]

I don't see the difference between one 600 cfm 4bbl and three 200 cfm 2bbl carbs.

As far as I can see, the only benefit of multiple carbs is fuel economy. You usually set up the throttle linkage so that only one carb is opened up at lower throttle applicaitons, and then they all open fully when you really get into it. The power of a big carb with the economy of a little one. That's pretty much the same thing GM tried to accomplish with the Q-Jet.

Well, they do look damn sexy too.

 

[slimer90210]

you're right. a single carb per cylinder (probably motorbike carbs?) would probably give better flow, but to keep it in an ideal state of tune you'd need to tinker around with it every second weekend. in my opinon, a few hp at the top end isn't worth that much effort. sidedraft webers have two throats, so having one throat per cylinder (3 carbs on a straight six) is almost the same thing, and a hell of a lot more friendly to drive and to maintain

A good Point. Keeping more then 2 bike or snowmobile carbs matched can be a real pain. But the throttle responce they give throughout the rpm range is unmatched, even by the best Webers and many EFI systems. My friend's dad runs a 1999 Neon with 4 45mm Mikuni Carbs and it has throttle responce like you wouldn't believe. He tried a good EFI setup and Webers but those little Mikunis just get the job done

I don't see the difference between one 600 cfm 4bbl and three 200 cfm 2bbl carbs.

At top end there is likely no difference, but at the low end the 3-200s will give better throttle responcethen the single 600 because they have smaller venturi and build air speed faster.

 

[Super Jamie]

I don't see the difference between one 600 cfm 4bbl and three 200 cfm 2bbl carbs

manifold design. three carbs would probably allow for straighter passage of air to the head, giving better airflow and more power in the top end. both carbs would atomise fuel differently so there may be changes there. and as the above poster said, airspeed throttle response

 

[slimer90210]

...as the above poster said, airspeed throttle response

Thank you Jamie....

 

[Gil]

Hey guys, I was just wondering if you could give me information about carburetors.
- What are the different types of carbs
- How do they work
- Tunability
etc.

I know it might seem like alot but leave nothing out, I'm a fast learner when it comes to cars and I'll read every bit of info you give me.

Thanks

This question is pretty broad. You've gotten good answers from the guys.
Do you have a specific application in mind?
A VW flat four is a bit different to carbeurate compared to a big-block Ford, Chevy or Dodge.
For Dodge/Mopar, one of the best carb systems was dual-quads. That is, 2-4bbl carbs on engines such as the 426 Hemi, the 440 wedge, and even the "little" 383.
With two huge 4 bbls you got good throttle response in the lwo end, as long as you didn't put your foot in it till you got some RPM built up. At the proper RPM point if you tipped your foot into it the back carb opened up burning up incredible amounts of fuel, and making incredible amounts of power.

With something like a VW in a drag racing context, the big Weber carbs can only feed the motor so quickly, but keep the thing from leaning out (running out of fuel) at the end of the track.

In most cases, especially if you're working on something that was manufactured in the "smog" era there are several types of Fuel injection that are superior, in most applications, to carbs.

 

[Velocity]

This question is pretty broad. You've gotten good answers from the guys.
Do you have a specific application in mind?
A VW flat four is a bit different to carbeurate compared to a big-block Ford, Chevy or Dodge.
For Dodge/Mopar, one of the best carb systems was dual-quads. That is, 2-4bbl carbs on engines such as the 426 Hemi, the 440 wedge, and even the "little" 383.
With two huge 4 bbls you got good throttle response in the lwo end, as long as you didn't put your foot in it till you got some RPM built up. At the proper RPM point if you tipped your foot into it the back carb opened up burning up incredible amounts of fuel, and making incredible amounts of power.

Yeah but you can't just slap dual quads on a stock engine especially a 383! It's to much fuel for it and contrary to what you're saying this will cause low speed flooding. The Hemi is the only Chrysler engine to come with Dual Quads straight from the factory because the the Hemi could handle the extra flow. It does after all, have the Hemi head design with huge valves which is the best design for drawing in and swirling the air fuel mixture.

A stock 383 with two carbs will flood badly unless they're smaller CFM carbs than the Hemi takes. Even a 440 six pack when set up stock doesn't quite have the appetite to take a WOT from the three two barrel carbs without some flooding.

Consider what happens when you pour to much gas into the engine:

You start to wash the oil off the cylinder walls and damage them. So you can't just slap on multiple carb's and expect everything to be hunky dory and faster.

There's and ad in a magazine I've got saying "Pull off that 4 barrel and slap on a six pack!" Yes great advice if you want to damage your engine. Your engine has to be set up to actually make use of the extra fuel. Upping the compression and changing the cam and exhaust are crucial. That's part of the reason that the stock 4 barrel 440's were running a 10.1:1 compression and the 440 six packs were running 10.5:1.

I don't think you'll find many examples if many at all of multiple carbs on low compression engines. These are the worst type of eninge to multiple carb unless you really know what you're doing.

 

[High-Test]

This seems a little off topic, but I was at a parts swap meet today (got some E-T torq-Thrusts )And I saw an Intake Manifold for a Big Block chevy that had 8 mounts for 8 2 barrel carbs, was this even possible? I've seen Fuel Injection Manifolds with the 8 Velocity Stacks, but the manifold I saw looked nothing like the FI one. I imagine It would get pathetic Gas mileage.