Deciding which main jets to buy to tune your carburetor can be a
difficult task. You may not know what jet size would be a
good starting point for your combination. Why buy a bunch of
main jets based on a guess when you may not ever use them again?
By drilling out your stock main jet to larger sizes you will be
able to narrow down what jet size your carburetor needs.
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You will need a numbered drill bit set.
We purchased this set from Harbor Freight. The brass
main jets are easy to drill so you can get by with one of
these inexpensive drill bit sets. |
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| The first thing we do is match
the stock main jet to its corresponding numbered drill bit.
If your stock main jet is a 100 (stock size in a 22mm Mikuni)
you will find this jet size falls between the #62 & #60 drill
bit.
Run the engine with the stock main jet and take a plug
reading. If it's lean and you need to increase the size
of your main jet, drill the jet out with the next size larger
drill bit, which in this case is the #60 bit. Run the
engine again with the drilled out main jet and take another
plug reading. Repeat this process until you get a good
plug reading. See our "Carburetor
Tuning" section to learn more about plug readings.
Do not skip drill bit sizes because once you drill the main
jet too large, you cannot go back to a smaller size.
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You should use a drill press for drilling your
main jet. You will need a vice with jaws that will hold
the main jet perpendicular to the drill bit. You do not
want the drill bit to enter the main jet at an angle because
it could create an oblong hole in your jet. |
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Once you drill the main jet to a size that
gives you a good plug reading you can use the chart below to
determine what size the jet is. In the photo to the left
you can see the drill bit measures 1.19mm. This means we
have drilled our stock main jet to the equivalent of a 119
main jet. |
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JET SIZE |
MM |
INCHES |
NUMBERED DRILL BIT |
JET SIZE WHEN DRILLED |
| #66 |
.66mm |
.02598" |
#71 |
|
| #68 |
.68mm |
.02677" |
|
|
| #70 |
.70mm |
.02756" |
#70 @ .0280" |
71.12 jet |
| #72 |
.72mm |
.02835" |
#69 @ .0292" |
74.2 jet |
| #79 |
.79mm |
.0312" |
1/32" @ .0312" |
79.2 jet |
| #80 |
.80mm |
.03150" |
#68 @ .0310" |
78.7 jet |
| #83 |
.83mm |
.03268" |
#66 @ .033" |
83.8 jet |
| #89 |
.89mm |
.035" |
#65 @ .035" |
89 jet |
| #91.5 |
.915mm |
.036" |
#64 @ .036" |
91.5 jet |
| #96.5 |
.965mm |
.038" |
#62 @ .038" |
96.5 jet |
| #101.5 |
1.015mm |
.040" |
#60 |
101.5 jet |
| #105 |
1.05mm |
.0415" |
#59 @ 1.06mm |
106 jet |
| #110 |
1.10mm |
.043" |
#57 @ 1.11mm |
111 jet |
| #115 |
1.15mm |
.0455" |
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|
| #120 |
1.20mm |
.047" |
#56 @ 1.19mm |
119 jet |
| #125 |
1.25mm |
.049" |
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|
| #130 |
1.30mm |
.051" |
#55 @ 1.33mm |
133 jet |
| #135 |
1.35mm |
.053" |
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|
| #140 |
1.40mm |
.055" |
#54 @ 1.40mm |
140 jet |
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