Jetting 101Jetting is a tough thing for many people because it's part science, part guestimate, part trial and error. Nobody can tell you exactly which jets you need for your carburetor, especially if you've done performance upgrades. Here's a basic tutorial on how it works.
Jets are small brass fittings that go in the carburetor and have very precisely-bored small holes in them for fuel and/or air to pass through. The size of these holes will determine your fuel to air ratio, and therefore how rich or lean your mixture is.
Most scooters have at least two jets in their carbs. Large frame vintage Vespas have four: a main jet, an atomizer (mix tube), and an air jet make up the main jet "stack." There's a separate idle jet too. Each of these is offered in different "sizes" (really hole sizes). It's important to know what each means.
EditHow to properly jet your carburetor
First of all, if your bike is stock, stay with the stock jetting. It was set at the factory and should work fine for you. If you live near sea level, that is. If you're at higher altitude, the air is thinner so you may need to go with smaller jets to get the proper mixture. Try one or two sizes smaller as a starting point.
Jetting is mostly done by first guessing which size you might need, then trying it out and testing it. If you've just installed a performance exhaust, try one or two sizes larger than stock as a baseline. If you've done more extensive performance work, go bigger yet.
Your spark plug is your indicator for how you're doing. Start with a fresh plug and go for a short ride. When you return, remove your plug and look at the tip. It should be a mid to chocolate brown color. If it's black and sooty, you're running too rich and need less fuel. If it's white or salt-and-peppered looking, you're too lean and need more fuel. Do this twice: The first time, keep the throttle open at least 3/4 of the way. This plug reading will tell you whether your main jet is the right size. The second time, keep the RPMs at 1/4 throttle. This will help you determine whether your idle jet is the correct size.
Scooters without a needle on the slide: while you're riding if you experience bogging or poor acceleration, the idle jet is probably either too big or too small. Try adjusting the air mixture screw first (which is basically a fine adjustment for the idle jet). If you can't adjust it out, change sizes and try again.
Scooters with a needle on the slide: there is usually an adjustment for the needle height in the slide. This will determine the fuel mixture in the middle 1/3 of the throttle range. You can do a third ride test with the throttle at 1/2 open to determine this mixture. If it's too rich, lower your slide a notch. If it's too lean, raise it.
EditLarge Frame Vintage Vespa Jet Types
Main Jet stack: This is the larger screw-in brass jet, really made of three sections, that regulates fuel/air mixture at the top 1/3 of the throttle.
Main Jet: This is the small, cone-shaped jet at the bottom of the main jet stack. They come in various sizes ranging 82-165. Most vintage Vespas will use one in the 85-116 range, though performance-modded engines can drink a lot more gas and use much bigger jets. The larger the number, the more fuel will flow through.
Atomizer (mix tube): This is the center of the main jet stack, and it has small holes in the sides. The rating on this type is BE1-BE5. These ratings don't seem to have any rhyme or reason. For example, a BE4 does not necessarily let in more fuel than a BE2. Most people leave this stock and work with the main and air jets.
Air jet: This is the top of the stack, and the part that has a slot for a screwdriver. The hole that goes through it allows air into the jet to pre-mix with the fuel in the atomizer. These are available in 120, 160, and 185 sizes. The larger the number, the more air will flow through.
Idle jet: This is the smaller jet that screws in next to the main jet stack. It regulates fuel/air mixture at the bottom 1/3 of the throttle. A combination of the idle jet and the main jet handle the middle 1/3. If your idle jet is too big or too small, it can create a "flat spot" in the acceleration. Idle jets have two numbers. The first number is the size of the bore that allows fuel in, and the second is the size that allows air in. Available sizes are: 38/120, 42/160, 45/120, 45/140, 48/160, 50/120, 55/160. So a 45/120 jet would run richer than a 45/140, because the 120 allows less air to pass. To make things more complicated, some vintage Vespas came with "plugged" idle jets, which have no air hole. These are available in sizes 42 and 50.
EditSmall Frame Vintage Vespa Jet Types
Small Frame Vespas have only two jets: a main jet that the needle passes through, and an idle jet.
Main jets come in various sizes ranging 37-88.
Idle jets come in only two sizes: 42 and 45. Generally, Vespa 50 and 90 use a 42, and Primavera/ET3 use a 45.
EditStock main jet sizes:
Vintage Vespa:
VNB - 82
VBA - 95
VBB/VLA/VLB - 100
VBC - 86
V5A - 63
V9A/V9B/VMA - 74
VSE - 116
VNX - 100
VLX('85) - 98
VSX - 112/116