> OK, the second part is true.
It's all true.
> It is also true that you will "feel some performance improvement"
> just by doing the carb mods. That basically yields a "European model".
Even the European models are lean these days. U.S. EPA and Euro EEB mandates restrict low rpm emissions resulting in fuel starvation and weak performance from internal combustion engines. The shim and airscrew 'mods' [tampering
] get you a few clicks closer to a pre-EPA machine, a motorcycle that runs like they did prior to 1980.
> If there isn't a siginificant amount of restriction in the stock
> air filter (and how do you know what "many think it is"?)
Time in grade. Experience with new riders has shown the first thing they want to do, not having a real clue why it could matter, is to rip out the stock air filter. The stock filter was designed by the Suzuki engineers to breathe well enough, even when dirty, to pass enough air to allow the engine to perform throughout the intended rev range.
> ... how do you account for the fact that removing the filter altogether results in an engine that just barely runs ?? (without any other mods).
All air filters restrict airflow, they are a wall of mesh in the doorway. A carburetor is simply a venturi tube with a reservoir [float bowl], fuel ports, and a metering device [slide needle]. As the piston moves down on the intake stroke it creates vacuum in the tube drawing air through the air filter. Resistance to flow from the air filter causes the vacuum in the tube to draw fuel through jet orifices. Removing the air filter allows the negative intake atmosphere to normalize almost completely through the end of the venturi tube bypassing the primary circuit jet which enters the flow earlier than the slide needle. Said another way, without the air filter the engine runs lean.
Suzuki's engineers are smart guys and way ahead of us consumers with our roll-aways and hand tools. Using EGAs, dynos, a myriad of sensors and other monitoring widgets, they calculate proper fuel flow into the combustion chamber with the given air filter and port resistance, and expansion flow out of the head with the given pipe and muffler resistance. Randomly removing or replacing parts of the flow system alters the equation.