Thinking about drilling carb jets is usually something that happens late on a Saturday night when you've finally finished a big engine mod and realize the thing is running lean. Maybe you just threw a pod filter on an old bike or put a high-flow exhaust on your go-kart, and now it's coughing every time you crack the throttle. You go online to order a jet kit, see the thirty-dollar price tag and the five-day shipping estimate, and look over at your workbench. You've got a drill. You've got some brass. Why not just make the hole bigger yourself?
It's a controversial topic in the mechanical world. Some guys will tell you it's a hack job that ruins the precision of the fuel system, while old-school tuners will tell you they've been doing it for decades with nothing but a steady hand and a set of torch tip cleaners. The truth is somewhere in the middle. If you're careful, it's a totally viable way to get your engine dialed in without waiting for the mailman.
Why you'd bother drilling instead of buying
The main reason anyone considers drilling carb jets is immediate gratification. Tuning a carburetor is an iterative process. You change a jet, ride it, check the spark plugs, and realize you're still a little lean. If you have to wait three days for every single incremental change, a simple tuning job can take three weeks.
When you have a set of micro drill bits on hand, you can make those changes in ten minutes. It allows you to find the "sweet spot" of your engine's fuel requirements in a single afternoon. Another factor is availability. If you're working on a vintage Keihin or an obscure Mikuni clone, finding the exact jet size you need can be a nightmare. In those cases, taking a smaller, more common jet and resizing it is often your only real option to get the bike back on the road.
The gear you actually need
You can't just walk over to your heavy-duty power drill and slap in a 1/8th inch bit. You're working with tiny increments here—usually measured in tenths of a millimeter. To do this right, you need a pin vice and a set of wire gauge or metric micro bits.
A pin vice is just a tiny, hand-operated chuck that you hold between your fingers. It gives you the tactile feedback you need to ensure you aren't oblonging the hole or snapping the bit inside the brass jet. As for the bits, you'll usually want a set that covers the range from #60 to #80 (in wire gauge) or 0.3mm to 1.6mm.
Using a power drill is the fastest way to ruin a jet. Brass is soft, and a high-RPM drill will grab the metal and tear it rather than shaving it. If you use a hand-turned pin vice, you can feel the bit cutting, which helps you keep everything centered and smooth.
Understanding the "one-way street" of drilling
The biggest thing to remember is that you can always make a hole bigger, but you can't easily make it smaller. Once you start drilling carb jets, you're committed. Because of this, it's always smart to have a few "sacrificial" jets. Don't start with the only main jet you have left for your daily driver.
If you do go too far and the engine starts blubbering and blowing black smoke because it's too rich, there is a "hail mary" fix. Some guys will flow a bit of solder into the jet and then redrill a smaller hole through the solder. It works in a pinch, but solder is much softer than brass and won't hold up forever. It's better to just treat the drilling process like a game of inches—go up one tiny bit size at a time until the engine responds the way you want.
Sizing isn't always a 1:1 match
Here's where it gets a little tricky. If you have a #100 jet, you might think a 1.0mm drill bit is the perfect match. In theory, that's how many jet manufacturers (like Keihin) size their parts. However, a hole that is drilled by hand won't have the same flow characteristics as a factory-machined jet.
Factory jets often have a specific taper or "venturi" shape inside the bore to help the fuel atomize as it's sucked into the airstream. When you run a straight drill bit through it, you're creating a straight-walled tube. This can actually change how the fuel flows at different vacuum levels. It's not a dealbreaker for most hobbyist projects, but it's the reason why a "home-drilled 105" might not behave exactly like a "factory 105."
The physical process of resizing
When you're ready to start, find a well-lit area and maybe grab a magnifying glass. Secure the jet—not in a metal vise that will crush the threads, but perhaps in a pair of padded pliers or a small wooden jig.
Insert the bit into your pin vice and start spinning it slowly with light pressure. You'll see tiny curls of brass coming out. It's a good idea to drill from the "inlet" side of the jet to keep the exit hole as clean as possible. Once the bit passes through, don't just yank it out. Spin it a few more times to burnish the walls of the hole and ensure there aren't any burrs left behind.
After drilling, cleaning is non-negotiable. Even a microscopic flake of brass left in that jet will eventually find its way into a narrow passage and clog your carb at the worst possible moment. Hit it with a heavy blast of carb cleaner and then use compressed air to make sure it's completely clear.
Testing your handiwork
Once the resized jet is back in the carb, it's time for the "butt-dyno" test. Start the engine and let it get up to operating temperature. If you were drilling the main jet, you're looking for how the bike pulls at wide-open throttle. If it screams at high RPMs but used to bog, you're on the right track.
The "plug chop" is your best friend here. Run the engine hard through a couple of gears, then kill the ignition immediately and pull over. Pull the spark plug and look at the color of the porcelain insulator. * White/Chalky: Still too lean. You need to go up another bit size. * Tan/Coffee with Cream: This is the jackpot. Leave it alone. * Black/Sooty: You went too far. You're now too rich.
When should you avoid drilling?
While drilling carb jets is great for older bikes, lawnmowers, or project cars, I wouldn't recommend it for anything high-performance or incredibly sensitive. If you're tuning a multi-cylinder bike with four carburetors, getting all four jets exactly identical by hand is nearly impossible. A tiny variation in the hole diameter across four cylinders can lead to one cylinder running hotter than the others, which is a recipe for engine damage down the line.
In those cases, it's worth the money to buy a calibrated set. But for a single-cylinder thumper or a fun backyard project, the drill-and-test method is a classic rite of passage. It teaches you exactly how fuel demand works and gives you a level of control over your machine that you just don't get by clicking "Add to Cart."
Final thoughts on the DIY approach
At the end of the day, drilling carb jets is about problem-solving. It's about taking a piece of brass and making it work for your specific setup. Whether you're trying to compensate for a new air intake or just trying to get an old tractor to idle again, it's a skill that every shade-tree mechanic should at least try once. Just remember to take it slow, use the right tools, and keep plenty of carb cleaner nearby. There's a certain satisfaction in hearing an engine roar to life on a jet you "made" yourself—plus, you get to keep that thirty bucks in your pocket for gas.