Drill sizes for main jets ?

[Iowa883]

I need a drill bit size chart for Keihin main jets . Does anyone have one or
know a link to one ?
Thanks,
Iowa883

[Charlie Gary]

Post by Iowa883
I need a drill bit size chart for Keihin main jets . Does anyone have
one or know a link to one ?
Thanks,
Iowa883

Can you buy drills in such tight increments? Don't know myself, but you've
sparked some curiosity. Also, will a drilled finish be smooth enough, or
will you have to chase it with a reamer?

[h***@gmail.com]

A good hobby model shop sells super tiny drill bits bought a few for engine detailing on models

[krusty kritter]

Post by Iowa883
I need a drill bit size chart for Keihin main jets . Does anyone have one or

know a link to one ?

Mikuni and Keihin round jets are in percentage of 1 millimeter. If you have a
#100 round jet, the hole in it is
0.0394 inches in diameter...

OK, so a 0.040 drill is close enough for 1 mm...

As you increase the diameter of the orifice, the flow increases exponential,
because area = pi times radius squared...

So a #150 round main jet is going to flow much more than 150% of the fuel that
goes through a #100 round main jet...

Most tuners of inline-4 cylinder liter bikes cannot go from, say, a #110 main
jet to a #135 main jet without having the bike huff and puff black smoke...



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[Mark Olson]

Post by Iowa883

Post by Iowa883
I need a drill bit size chart for Keihin main jets . Does anyone have one or

know a link to one ?
Mikuni and Keihin round jets are in percentage of 1 millimeter. If you have a
#100 round jet, the hole in it is
0.0394 inches in diameter...
OK, so a 0.040 drill is close enough for 1 mm...
As you increase the diameter of the orifice, the flow increases exponential,
because area = pi times radius squared...
So a #150 round main jet is going to flow much more than 150% of the fuel that
goes through a #100 round main jet...

Aren't jets also contoured somewhat, rather than being a simple
cylindrical hole drilled through a block? IOW, the hole in the
middle is something like the hole in a donut, not a straight thru
hole drilled through a piece of flat stock.

I could certainly be wrong, but I seem to remember reading that
this is why you couldn't get the exact flow characteristics of
a larger factory made jet by simply drilling out smaller jet to
the same sized hole.

[krusty kritter]

Aren't jets also contoured somewhat, rather than being a simple cylindrical

hole drilled through a block?

True, they have a venturi shape...



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[Battleax]

Post by Mark Olson

Aren't jets also contoured somewhat, rather than being a simple cylindrical

hole drilled through a block?
True, they have a venturi shape...

I've never seen this. Every main jet, idle jet, I've delt with are simple
straight holes.
If a jet is not available in the size needed then a numbered drill is used
to enlarge the passage.
There's no reason to have a venturi shape in a jet.
B

[krusty kritter]

Post by Battleax
I've never seen this. Every main jet, idle jet, I've delt with are simple

straight holes.

Look it up on the web. If you find cross-sectional drawings of Mikuni round
main jets, you'll see that there is a venturi shape to the jet...

In fact, some Mikuni jets are called "reverse venturi" jets...

Post by Battleax
There's no reason to have a venturi shape in a jet.

At least, there is no reason that *you* are aware of ;-)



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[Battleax]

Post by Battleax

Post by Battleax
I've never seen this. Every main jet, idle jet, I've delt with are simple

straight holes.
Look it up on the web. If you find cross-sectional drawings of Mikuni round
main jets, you'll see that there is a venturi shape to the jet...
In fact, some Mikuni jets are called "reverse venturi" jets...

Post by Battleax
There's no reason to have a venturi shape in a jet.

At least, there is no reason that *you* are aware of ;-)
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You are correct on the main jet. Drilling such a main jet would be a bad
idea.