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How To Drill the Main Jets
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.
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.

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.

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.
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.
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"
#120 1.20mm .047" #56 @ 1.19mm 119 jet
#125 1.25mm .049"
#130 1.30mm .051" #55 @ 1.33mm 133 jet
#135 1.35mm .053"
#140 1.40mm .055" #54 @ 1.40mm 140 jet