Finding the right drill for jet size usually is really difficult cause. You do not always easily guess which jet size answers as a start point for any special mix. Rather than buy a lot of main jets according to a simple guess, you can bore exactly a jet until bigger size.
For instance, jet 178 require drill of 1.75 mm. Keihin-jet numbers and sizes do not always match perfectly with those of Mikuni. Mikuni and Keihin numbers lead to amount of fuel flow for minute.
How to Drill and Size Carburetor Jets
The numbered drills range of 80, the smallest, until 1, the biggest. Those numbers base on wire-measures, so big numbers point little drills. For little engines you commonly choose bits between 61 and 80 for carburetor holes.
Some uses even #60, that is 0.040 inches, for set flows holes on gas appliances. Other sample is #75 wire-measure drill in 0.021 inches, that answers for around 53.3 jet size. #36 drill hardly you find, but bit of 0.0145 inches is the nearest to #37 jet diameter.
Using handheld drill, you risk various troubles. The hole can exit off-centered or oval, what alters the planned size. When you use handheld drill, the hole commonly becomes a bit too big or not round.
Everything depends of the setup of the gear, the drill self and the accuracy of the chuck with cone. For 1.2 mm size even simple drill press can err in some thousandths. More well use lathe for such little holes.
If you bore manually, the bit can wobble and cause oval hole bigger than wanted.
Many drill sizes exist, with relations to inches and millimeters. For instance, #43 or almost 3/32 drill matches around 235 jet. 5/64 drill is 215 jet, while 1/8 drill comes to around 255 jet.
For inbetween jet sizes you can measure between those values. If you intend jet more than 200, twist to factory specialist for go until 250. During boring start by means of little size and expand step are more simply.
Help to do plug readings, for control, when you reached the right jet size.
