Quote from: MotorGeek on April 07, 2014, 10:22:16 pm

An air flow meter and a precise fuel flow gage like we use on the dyno will not tell you when you have the mixture that makes best power. 

Not sure I understand this one. So why wouldn't it?

The airflow and fuel flow meter will tell you how much air and fuel is passing through the engine at any given RPM.  Using this information, a theoretical air/fuel mixture can be calculated. 

Not all of the air and all of the fuel that passes into an engine will make it into the combustion chamber.  This statement holds true for 2 and 4 stroke engines.  Some of the raw air and fuel passes through the engine during the scavenging phase into the exhaust system and may not get a chance to experience the combustion process.  How much of the air and fuel are trapped in the cylinder each engine revolution is known as the engine's trapping efficiency.  An engine that has 100% trapping efficiency does not allow any fresh air or fuel to escape into the exhaust system.  The trapping efficiency on most two strokes is very poor.  On a two stroke engine, the trapping efficiency is usually the highest (best) at the torque peak and the worst at the lower RPM where the torque dips just before the engine “comes on the pipe”..

The exhaust system and the ports are having a serious mismatch at the RPMs where the torque dips just before the engine “comes on the pipe”.  At this transition RPM, there is a lot of fresh air and fuel passing through the engine into the exhaust pipe while much of the exhaust has not been scavenged from the cylinder.  At this transition RPM the airflow and fuel flow meters may show the air/fuel ratio to be theoretically optimum, the O2 sensor is often showing off the scale lean and future jetting changes will show a power increase in this RPM range when the main jet is severely leaned down.  By trial and error, the main jet that produces the best power can be found using the dyno or other performance measuring devices..

Every two-stoke engine design has a different personality (trapping efficiency, combustion efficiency, scavenging efficiency and thermodynamic efficiency).  The design of the exhaust system and how well it works with the rest of the engine design has a major effect on these listed efficiencies.  It is next to impossible to determine what these efficiencies will be when the engine is on the drawing board, but can be determined with sophisticated test equipment once the engine is produced.  Much can be learned about different design efficiencies within a particular engine by looking at the exhaust composition with a multi-gas exhaust analyzer.  A series of dyno runs on different engines using different jets can show what jet will produce the best power.  The unfortunate truth is the optimum jet on each of the different engines will not produce the same numbers from the measurements taken with an O2 sensor, a 5 gas analyzer or the air flow meter and fuel flow meter.  It still requires field-testing, dyno testing or some other method to gage performance to find the jet that produces maximum power.