Re-jetting via drill...part II

[ManagerMike]

At the end of the last thread I opted to buy instead, well my new 145 mains just got here and for sh*ts and giggles I used my 1.45mm drill as a pin guage and it fits rather loosely.
So it seems that the article in the Tech Tips about drilling jets may or may not apply to other carb types, but does not apply to our carbs.

Just thought I'd share, Oh and when I get around to the actual replacing of said jets (sometime over this winter) I'll use the same drill to check the fit on the original (as far as I know) 137.5 mains, just for the halibut, and report on that.

[81xsproject]

At first, I didn't give this topic much thought. However, with some idle time this morning and a hot cup of coffee in hand, it all makes sense. The jet sizes control the flow rate. So, why in the world would anyone size them in constant increments of diameter when the flow rate is relative to the area (an exponential function) in the opening of the the jet. If they did size jets in constant increments of diameter, every step change in jet would be a different flow rate change than any other step. An example: I will look at changes in flow/area relative to jet changes by diameter from a 1.375mm to a 1.40mm, to a 1.425mm, and to a 1.45mm.

1.375mm/2= a radius of .6875mm so (3.14)(.6875^2)= 1.4841 square mm
1.400mm/2= a radius of .7000mm so (3.14)(.7000^2)= 1.5386 square mm

So, this .025mm diameter jet change would be a difference of .0545 square mm of area.

1.425mm/2= a radius of .7125mm so (3.14)(.7125^2)= 1.594 square mm

So, this .025mm diameter jet change from 1.4mm to 1.425mm would be a difference of .0554 square mm of area.

1.450mm/2= a radius of .725mm so (3.14)(.725^2)= 1.651 square mm

So, this .025mm diameter jet change from 1.425mm to 1.45mm would be a difference of .0565 square mm of area.

So, the point of all this drivel is that if jetting sizes were done by hole diameter, it would be very difficult to re-jet because every jet change would be different than every other jet change as you can see in the example above.