Jet Size vs Gas Flow Rate

jaguar

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#1
I've read that there are significant differences between the different brands of jets due to the entry and exit angles for their holes. But when I tested a bunch of main and idle jets for flow rate they all basically were the same, with the flow rate only depending on the hole size (the area to be exact). I tested them using an inverted water bottle filled with premix and determined the flow rate by how much time they took to fill a certain volume. The graph for the idle jets was a bit offset from the main jets.
 

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jaguar

Subscriber
Joined
Jul 29, 2000
Messages
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69
Location
South America
#2
I also bought a Mikuni needle jet and tested the flow rate through it with the needle at different locations in it. At "closed throttle" with the fattest part of the needle at the needle jet (allowing the most minimal space) there was still some flow. That means the needle width in relation to the jet diameter has an influence on the needed idle mixture. If the needed # of idle jet is high then the needle should be skinnier, and if the needed # of idle jet is low then the needle should be fatter or the needle jet is wallowed out (something that happens mostly with 250cc and bigger). Note the graph doesn't show a linear relationship between the flow area and the flow rate like we have with the idle and main jets. That means there are boundary layers that restrict the flow. That is to say that the zone close to the surface flows slower due to a type of friction. In the graph below the needle jet flow area is the varying area between the needle and its jet (the hole it slides in).
 

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