History, theory and evolution of the caburetor.

I bought an old carburetor from ebay -cheapest for five dollars. I pretty sure it came of of a cb750. I will take it apart and put some photos up and ask some questions.







just to go over some basics - in my mind carburetors create gas fumes that are sucked into the piston chamber via a vacuum that is created many ways. The diaphragm helps create a vacuum; when the valve is pushed down (opens) and the gas fumes get sucked in from the piston motion of coming down. The fact that the air filter opening is larger than the opening leading to the chamber also creates a vacuum. The more you open/turn the throttle the more gas fumes are created and sucked it to the chambers to be ignited.

 

Okay - that's the diaphragm. One thing is I couldn't blow through that little needle. Is that suppose to be clear. Have diaphragms been around since carburetors were created.

 

Good deal on diving in with a reference model.

If you looking to add to your library shelf Mike Nixon's Cleaning the Honda V4 Carburetor is a good read.

Here is a link:

The Motorcycle Project Book Nook

 

my question is the reason for these little holes (two and three) and do they need to line up to something when putting it back together.

 

thank you!

 

i found this very helpful years ago
Carburetion 101

 

thank you for the link.

 

karl the dudes reference page is a good one. Carbs create a fine mist not a vapor. Unless you get vapor lock caused by heat. There are other carb reference sites out there. You just have to look. SabMag is one. http://users.rcn.com/kochc/moto/carb/carb.html

A simple way to think of a carb is it's nothing but a controlled drip device. do research on the first ones and you'll see what I mean.

 

I have two new questions:

1) when a carburetor is listed as a 28mm what is 28 mm. My thought is the narrow spot on the carburetor where the speed of the air picks up;
2) it seems that just a little more screw does a lot two a carburetor - so for example just 1 or 2 or 3 extra mm of steel can affect the whole mist/mixture and that affects the way the bike runs.

 

Bernoulli is responsible !!

Read "Weber Carburetors: Tuning and Maintenance" by John Passini. All the wisdom applies to bikes too.

There are usually 4 different circuits (sometimes 5) that combine to supply combustable air/fuel ratio mixture to engine at different rpm's, and each is different.

 

there are only two reviews on that book on amazon and they are split.

 

I like the holley 650 double pumper....slap that bitch on a motor and it helped you pass everthing..........but the gas stations.LOL

 

Yep. That's called the "venturi", BTW.

 

The 28mm refers to the circumfenece of the carb part the slips inside the carb boot and is help in place by that round o-ring thingy with the screw on it.

Sorry, brain lock on the correct terms.

BZ

 

If I could recommend an additional book for the library, this a great read with in depth coverage on all the various components/systems on a motorcycle.

Qbookshop.com - Sportbike Performance Handbook

 

But I.C., all those bikes on the cover are fuel-injected !?

 

Reviews?? Screw bad reviews; no one could read this book with out learning ALOT !!

And you'd need to vet the credentials of the reviewers in order to evaluate the validity their opinions.

 

Well noted. I have a First Edition copy of this book which covers carburetors. I'm not sure if the Second Edition does or does not.

 

The diaphragm controls the height of needle, therefore controlling the amount of fuel that is drawn into the venturi area via, you guessed it, the venturi affect. The restriction created by the venturi increases the speed of the air traveling through that section of the carburetor body. The faster moving air increases the venturi affect on the fuel waiting to be drawn up through the jet from the fuel bowl. This vacuum also acts on the diaphragm, pulling it and consequently the needle up to allow more fuel to flow from the fuel bowl. Inside the fuel bowl is a float that simply maintains the level of the fuel in the bowl at a neutral level were it can be drawn into the engine. Engines are basically pumps, pulling air in one side, and pushing it out the other. When the throttle is closed(idle) there is not enough air flow through the venturi to draw fuel into the engine. This is where the idle and progression holes and circuits come into play. They are next to the throttle butterfly and bleed fuel into the engine at closed and partial throttle. These are the most difficult parts of the carb to tune. It is easy to tune the engine to run well at wide open throttle by simply changing the main jet. Tuning the idle jets and emulsion tubes(some carbs) to add the correct amount of fuel when the throttle is being opened from close to around 1/4 throttle when the main circuit takes over can be extremely difficult. Hope I could give you a little insight. For a real simple design, go pull the carb off your lawnmower and clean it out(probably needs it) I've cleaned dozens of those things for neighbors, friends, and relatives who couldn't get their power equipment going in the spring. When all is said and done, you'll marvel at the invention of fuel injection... ahhhh

 

OK, good explanation, but washers under needle or slots on the needle (another adjustment) also as well as throttle slide control the mid-range a/f mixture, and emulsion tubes don't factor in at lower speeds.

As many advantages as fuel injection has over carbs, there are many advantages of carbs over injection, and the debate will continue.