Some common misconceptions about carburetor tuning....
....This is to address some common myths / misunderstood concepts I get questions on routinely. Number one on the list is "I'm going to jet my carb up and make it bigger" No, sunshine, you are going to make it richer-follow along for a minute.This is greatly over simplified and does not take in account things like internal passage friction, air /fuel density, etc,but should serve to demonstrate the principle. Most people understand air fuel (A/F) ratios nowdays, so lets assume your carb as jetted produces 13:1 at WOT (near optimum) and 14.7:1 at cruise, known as stoimetric or the theoretical perfect mix.Understand first, the venturi will only allow so much air to pass at atmospheric pressure. In other words, like a bucket full of sand, no matter how much you shovel in, it will hold (or in our case flow)only so much(imagine supercharging as packing the sand instead of just dumping it in).Figuring jet sizes is quite complicated, but basically a certain percenatage of the venturi size is used in the jet to provide the proper amount of fuel to the air going thru to cause the desired ratio-making the jet bigger adds fuel,but no more air.Along these lines it is good to understand that a 100 jet is not necessarily 10% bigger than a 90.Jets are round and you must use area to figure true size differences-remember back in school-area of a circle is "PIE R Squared" or 3.14* (radius*radius).Example:most people will tell you to go up 8 or 10 jet sizes when removing the power valve(see other carb aricles for info on this). Let's say we are starting with 86 jets and go up to 94 -somewhere between 9 and 10 % right? Wrong-if you figure area on both you will see it is more than double that-around 19.6%- remember that when using A/F meters for tuning.Looking at it a different way to see what happens when you "jet up", use the same example, only leave the power valve in. If we had 13:1 WOT before, now we have 10.86 (WAAAYY rich) and the 14.7 cruise becomes 12.29 or richer than we want for WOT-(this explains why it "feels right" when hammered, but runs rich everywhere else.) Yes, for those of you deeper into this, the explanation is not exactly accurate, but it is meant to be understood easily. Another, not so common, issue is air bleeds. These are not meant to be used instead of jets. One reason is that they do not control just mixture.The fuel from your bowls is maintained at a specific level. This affects when the main circuits start and stop flowing. Imagine the air rushing down thru the venturi -the low pressure area created is where the booster is generally placed. The idea behind this is twofold-one with proper shaping, it can help with atomization.The other, more importantly for this conversation,is to produce "suction" to pull the fuel into the airstream.Now, to backtrack a bit, the air bleed sits on top of a vertical passage-one end of this is fed by the float bowl and the other goes to the booster. The level in this passage is roughly equal to that in the float bowl. Now, keeping that in mind and digressing bit-you can use higher float levels (very slight ) to quicken the main circuit startup and lower levels to slow response and to some degree lean the mixture. Back to the air bleed- it is there for more than one reason also. It serves as an air break when you shut off the engine in order to keep the booster circuit from siphoning the mixture while the engine is stopped. Also, as the fuel passes thru, some of the air in the bleeds mixes with the raw fuel (known as emulsification). This modifies the weight of the fuel and causes quicker or slower reaction as well as more or less fuel to be pulled out depending on the amount of air (size of air bleed has an effect on this)-again, a very rudimentary and not exactly perfect explanation. Herein lies the problem with making radical changes in air bleed sizes- keep in mind the explanation above on areas of circles- if you go very far rich or lean, you also cause the main startup to be altered and can cause avery poor flow curve in the carb- this is why you don't want to go more than 3 or 4 sizes either way-use the jets to tune and the air bleeds for track fine tuning, or if you want to get very scientific, you can optimize jetting and use air bleeds and float levels to tune your curve. This circuit is one of the main ones to consider when thinking about booster changes- too big and you restrict the carb,too small and you'll have a real flat backwards curve-also keep in mind the internal passage sizes differ and can affect flow curves greatly..... I know this is pretty brief, but it addresses some pretty important areas I think.
