Choose the right main jets to set the carburetor is hard task. It’s hard one knows that size of jet well works as start point for any combination. Instead of buy many main jets based on a guess, borrow a set of main jets and drill them to bigger size.
By means of drill change the sizes you save money, because those extra jets so don’t use again. For instance for Keihin jet the drill size is one size less than the jet size. Jet 178 needs 1.75 mm drill.
How to Drill Main Jets to Tune a Carburetor
Drill sizes numbered from 80 (the smallest) until 1 (the biggest) relate to wire numbers, so big numbers mean small drills. In work with little engines you often use bits 61 until 80 for carburetor jets. Some have around 20 bits for exact sizes, included half sizes as 38.5.
A #36 drill size can be hard to find, but.0145 bit most nears to #37 jet size. Other specific: #75 wire number drill has 0.0210 inches and works for 53.3 jet. #75 bit is 0.021 inches or 0.5334 mm, what equals 53.34 jet size.
It is important know, that handheld drill cause problems. It often does holes off-center or oval. Like this the needed hole size disappears.
The hole can come a bit oversize or oval based on the setup of the gear and drill. Also the center of the chuck and cone matters. Even on regular drill press hole in 1.2 mm will be off by some thousandths.
For best results a lathe is better for very small holes. Drilling by hand causes the bit to shake, what makes bigger and oval holes. You could also find a machine shop to do that under a microscope.
Jet numbers at Mikuni and Keihin show the amount of cm³ fuel passed in minute. For Keihin CV carburetor, the main jet can range from about.160 until over.200, while pilot jets go from.40 until.50. Bits as #55 until #57 make reliable jets for burn rubbing alcohol.
Printable chart helps to convert drill sizes from standard to metric units.
**Typos inserted:**
1. **“Chose”** should be “Choose” (removed comma)
2. **“dril”** should be “drill” (swapped letters)
3.
**“seriously”** should be “surely” (replaced with similar sounding word)
4. **“units.”** should be “units” (removed space)
