Dave has a very good question and when people first sat down and designed carburetors they probably hoped the things would work as Dave has suggested and to some extent they do.  Dave is suggesting that a linear relationship exists between fuel flow and air flow, if one doubles, the other doubles etc.  But as Boule'tard has stated it doesn't quite work that way.

Air constantly changes in density as it flows which is why modern cars have detectors to measure air mass flow so the computer can determine how much fuel to inject.  Air is a compressible fluid and fuel being a liquid is not compressible, they behave differently.  If you double the flow through the venturi you will draw in more fuel through the jet but not necessarily at the same increase in rate.  The two do not follow a straight line if you were to graph air flow vs. fuel flow.  Also in combustion it is the mass of the fuel and mass of the air that count whereas in a carburetor it is velocity and pressure that do the metering and so the carburetor only indirectly meters out the correct mass of fuel for a given mass of air.

Having said all that, Dave is essentially correct and even simple carburetors do a reasonable job, look at the carb on an old "hit and miss" engine.  At one time whatever made an engine run was considered good enough.  Then people started asking the carb to be efficient under all conditions and it has become more complex trying to do it.  That's why the good old carb is almost obsolete and is being replaced by fuel injection.

Boule’tard wrote on 05/09/12 at 13:27:54:







OK, here's my take on the whole mod it to make more power thing.  

We tell them to rejet.  

We tell them to put on free flowing muffler.  

We tell them to put on a free flowing air filter.

You guys are getting tied up in your knickers over the order of things like A causes the need for B or C.    False assumption there, you can do them in any order but you will likely wind up doing all of them as they complement each other's improvement effects.

Jetting generally gets tossed to the end of the bus since you fine tune the jetting AFTER you finish muffler and filter mods (or you get to do it two or 3 times as some of us have done)

I wish carbs were that good as to be totally self adjusting -- they aren't.    Sorry.

Good thread indeed,....I think Engineer said it best'

A carb is designed to work exactly like Dave said, but like was said, a carb is not perfect.....like EFI is much closer to being.

This is the main reason I always advise not to rejet, untill there is an obvious reason to do so, and then to do it one step at a time.
W/ some here, there is a bad mind set that x mod = x jet. Its just not that cut and dried. Many times no rejetting is required after a muffler mod,.....and that just mite be because the carb really did do what its designed to do.