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Carburetor Jetting 101

Carburetor Jetting 101

Getting the most out of a scooter’s carburetor jets is a tough thing for many people. Carb jetting is often part science, part guesstimate, part trial and error. Without a dynamo-meter and some pretty advanced tools, nobody can tell you exactly which carb jets will extract the maximum performance from your hardware, especially if you’ve created a “one-of-a-kind” machine with a series of aftermarket performance upgrades.

That doesn’t mean there’s nothing to learn and that you can’t – through trial and error – find the right jetting on your own. With that in mind, here’s a (very) basic tutorial on the theory behind proper jetting.

Don’t forget: if you can’t find what you’re looking for here, don’t hesitate to call Scooterworks’ customer support at 1-888-968-3772.


Dellorto Carb Jet

Carburetor jets are small brass fittings (above) 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.

If your bike is stock, Scooterworks recommends original, factory-spec carb jets. Those settings were selected at the factory for maximum drivability and emissions compliance, and should work fine for you. If you live near sea level, that is. At higher altitude, the air is “thinner” (less oxygen dense) and with smaller jets may be required to get a proper air/fuel mixture.

If you have a factory-spec. scooter and spend most of your time in a high-altitude riding environment (Denver, Mexico City, etc.) start out by trying a jet that is one or two sizes smaller than the factory “sea level” jet.

If you’ve just installed a performance exhaust, try a jet that is one or two sizes larger than stock as a baseline. If you’ve installed a Malossi cylinder kit on your Stella (for example) or done other extensive performance upgrades, go bigger yet!

If you can’t tell whether or not you’ve selected the right jets based on your riding experience, your spark plug can be an effective indicator of how you’re doing. Start with a fresh plug and go for a short ride, then remove your plug and check the tip, which should be chocolaty-brown color. If it’s black and sooty, you’re running too rich (the jet is too big) and need to trim back the fuel (with a smaller jet). If the plug’s tip is white or “salt-and-pepper”, you’re running too lean (the jet is too small) and need more fuel.
TIP No. 1: with your new jet installed, run your scooter’s throttle 3/4 of the way open, then check the plug.
TIP No. 2: after you’ve tried No. 1, repeat the process at 1/4 throttle to determine if your idle jet is the correct size.

For scooters with a needle on the slide, there is usually an adjustment for the needle height in the slide which determines 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.

For scooters without a needle on the slide, the adjustment process is a bit different. If you experience bogging or poor acceleration while you’re riding, the idle jet is probably the wrong size. Try adjusting the air mixture screw first (which serves as a fine adjustment tool for the idle jet), and if the problem doesn’t “adjust away”, change the jet size and try again.

Understanding proper jet selection for large frame Vespas requires identifying a number of components within the carb, which include …
– the larger screw-in brass jet (made up of three sections) that regulates fuel/air mixture at the “top” 1/3 of the throttle range (66-100% throttle).

– the small, cone-shaped jet at the bottom of the main jet stack. These are available in various sizes, ranging from 82-165. Most vintage Vespas will use one in the 85-116 range, although high-performance and race-tuned engines may require much more fuel and use much bigger jets. The higher the number, the larger the jet and the more fuel will flow through to the engine.

ATOMIZER (mix tube)
– located in the center of the main jet stack, 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.

– located the top of the stack, with a slot for a screwdriver to make fine adjustments. 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. As before, the higher the number, the larger the jet and the more fuel will flow through to the engine.

– the smaller jet which is screwed in next to the main jet stack, the idle jet regulates fuel/air mixture at the bottom 1/3 of the throttle range (idle-33% throttle opening). A combination of the idle jet and the main jet handles the middle 1/3 (34-65% opening). If your idle jet is too big or too small, it can create a “flat spot” in the scooter’s acceleration.

Unlike the other jets on this list, 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. Some common idle jet 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).


Small Frame Vespas have only two jets: a main jet (that the needle passes through), and an idle jet. The small frame’s main jets are available in various sizes ranging from 37-88, but the idle jets are available in only two sizes: 42 and 45.

From the factory, Vespa 50 and 90 use a size 42 idle jet, while the Primavera and ET3 models use a 45.

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