jaguar
~SPONSOR~
I'm pretty sure that every other graphic you have seen showing the amount of jetting effect each adjustable part has was only some persons idea coming from their own experience, not from a computerized jetting calculator. And none of it was "specific", only generalized. This graphic comes from my jetting calculator and is specific in that the amount of change is due to +/- 20% change. For example, if the main jet is a #100 than the graphic shows the change from 80 to 120. 20% change for the needle was from 1.2 degrees to 1.8 degrees taper.
How a main jet change affects idle richness: At closed throttle the flow are around the needle (between it and the needle jet) is about half of that of the main jet. So it is an open flow area that is being acted upon by the suction in the carburetor. Its gas flow adds to the idle jet flow. Although the main jet area at closed throttle is twice the flow area around the needle, it still affects the flow by presenting some flow resistance. So any main jet change will somewhat affect the idle richness. My calculator says a 20% main jet change creates an 11% idle richness change which is a 2:1 ratio. So if you increase the main jet by 2% (a typical change) then you affect the idle richness by 1%. Of course that is hardly anything which is why they drew the graphic the way they did. But still it is erroneous. I need to recalculate everything to show typical changes.
How a main jet change affects idle richness: At closed throttle the flow are around the needle (between it and the needle jet) is about half of that of the main jet. So it is an open flow area that is being acted upon by the suction in the carburetor. Its gas flow adds to the idle jet flow. Although the main jet area at closed throttle is twice the flow area around the needle, it still affects the flow by presenting some flow resistance. So any main jet change will somewhat affect the idle richness. My calculator says a 20% main jet change creates an 11% idle richness change which is a 2:1 ratio. So if you increase the main jet by 2% (a typical change) then you affect the idle richness by 1%. Of course that is hardly anything which is why they drew the graphic the way they did. But still it is erroneous. I need to recalculate everything to show typical changes.