Question about carburetors..

[Rafs-T-Type]

I really don't know much about how they work. My question is how is the fuel amount controlled to each cylinder. How does it keep the amount even? What would cause some cylinders to be lean and some rich?

 

[Dusty Bradford]

It can't which is why efi is preferred. In a racing carb'd engine you use things like a high rise single plane intake and carb spacer to get the carb up higher so the intake can more evenly distribute the fuel air mixture.

 

[87GNSteve]

Right, it all has to do with manifold design, AND carburetor fueling loves heat, which aids in fuel vaporization and retards puddling.

 

[John Larkin]

Carbs basically leak fuel into the air stream caused by vacuum in the engine from the intake stroke; the engine pretty much sucks it in. The leak is governed by all kinds of fuel circuits, venturi size, jet size, throttle blade orientation, number of barrels, etc. It's really high tech but not in an electronic way. The intake has more to do with fuel distribution than anything. That is why most street cars work best with a dual plane which is what most OEM intakes were.

I don't know if carbs love heat that much; we used to run carb isolator plates, big sheetmetal pans under the carb to keep intake heat away. And the fuel lines certainly don't like it - vapor lock.

 

[whitehot84]

They work best on Pinto's and Vega's.

 

[novaderrik]

a well tuned carb can rival efi in it's ability to efficiently meter fuel.. it gives the engine what it wants, not what the ecm thinks it needs.

 

[charlief1]

a well tuned carb can rival efi in it's ability to efficiently meter fuel.. it gives the engine what it wants, not what the ecm thinks it needs.

Thank you Derrik, the rest of you suck to say the least. It really boils down to the design of the intake and the carb. It really doesn't matter if it's carbed or injected (insert some sort of sex joke here) both can work well if they're done right. The key word is right here. You have to match the CFM of the carb to the engine to some degree but it's all about jetting the carb and getting the right springs for the vacuum level. If everything is equal a carb will give an injected engine a run for it's money.

 

[groumoutis]

Charlie said injected......hehe.

Sorry, you ask for something and that's all I got for now.

 

[charlief1]

Charlie said injected......hehe.

Sorry, you ask for something and that's all I got for now.

Well I'd rather be injecting than injected.

 

[Dusty Bradford]

HP for HP a carb can be just as good as EFI. My problem with it is EFI and laptops have spoiled me. All the info is now on the screen in front of me and jet changes are made with a couple key strokes. My 69 Camaro still has an old Holley on it that burns your eyes at idle, but it's a muscle car so I don't mind. lol

 

[Rivical]

HP for HP a carb can be just as good as EFI. My problem with it is EFI and laptops have spoiled me. All the info is now on the screen in front of me and jet changes are made with a couple key strokes. My 69 Camaro still has an old Holley on it that burns your eyes at idle, but it's a muscle car so I don't mind. lol

Holy crud! Tune that thing!

Carbs are more hands-on, plain and simple. Having the right intake becomes of great significance because of the air/fuel requirements. But if you know your combo, they're just as easy to tune for conditions. Just that you actually have to pick up tools and manually perform the action's of change. It's a matter of taste IMO. There's no reason that a good tuner, in either carbed or injected preference should have an advantage because of that choice alone in a heads up race with static settings.

 

[Rafs-T-Type]

So back to my original question, what would cause only some cylinders to be lean? Could it be weak compression causing weak vacuum in that cylinder to pull the mix in? Or a blocked up intake runner?

 

[charlief1]

On a carbed engine most of the time a lean condition is on all cylinders, not just 1. If you've got only 1 lean then it's either a bad intake design or something blocking that specific cylinder. It could have crud on the valve or some sort of lift issue.

 

[Rivical]

Is this a TR or what are we talking about? Stock manifold? Specific vehicles have known problem areas. It might help to know what you're working on.

In addition to Charlie's notes, it could be a worn cam lobe or worn, weak lifter/s.

 

[novaderrik]

most intake manifolds don't get totally equal air/fuel distribution to all the cylinders. actually, most heads don't give equal flow to all the cylinders... throw in a worn cam lobe and a valve that doesn't seal properly- and an exhaust that doesn't scavenge all the cylinders equally- and you will have some cylinders not doing as much as the others.
you can compensate for different air/fuel ratio from cylinder to cylinder by staggering the jetting and adjusting idle mixture screws- and some higher end carbs have idle mixture screws on all 4 corners to help even things out, too.

 

[Rafs-T-Type]

Thanks for the info guys.