A while back over in another thread, I threw together a quick n' dirty hand drawn jetting chart. I finally got around to producing a cleaner version and wanted to share it here, along with some basic guidance on how to jet an SI carb, because this comes up all the time.
As always, feedback is appreciated, because while I'm not bad at this, there are always others who are better.
Next, let's make sure we're all using the same terminology.
The main jet stack:
And the idle jet:
As you look down at the carb in the air box, they are located like so:
There are, for our purposes, only four jets and one adjuster screw in play in an SI carb. You can also change out the carb slide, but that's usually not necessary for any but the most aggressive tuning setups. I'll briefly touch on the theory of it, though.
Each jet has a different throttle range in which it's effective, and how much it contributes also varies across the range. Here's a chart which lays it out.
1) The Main Jet (MJ) -- The little jet at the end which mainly impacts Wide Open Throttle (WOT) and which people just getting started with jetting seem to think is the only jet that matters.
Main jets are numbered and usually are in a range of about 90 to 140, with stock motors usually being in the 98-110 range. The number refers to the size of the opening in the tip of the jet in hundredths of a millimeter, so a 100 MJ is, in theory, 1mm in diameter. As a general rule, though, don't assume that value is accurate unless you've checked it with some sort of bore measure, although if you buy a set of jets from a reputable manufacturer or source, they will probably at least be consistent with each other.
Adding go-fast parts like exhausts and performance cylinders usually requires "upjetting," meaning to install a larger main jet.
2) The Atomizer -- also sometimes referred to as an "Emulsion Tube" or "Mixer Tube" -- Primarily effects the mid-range (1/4 to 3/4 throttle). This jet gets ignored at your peril, because once you get up to speed, if you pay attention you'll realize that you're probably cruising at 1/4-1/2 throttle
Atomizers are NOT numbered in sequence of richness. They are numbered in the order they were first produced. The order is is as shown in the chart from leanest (left) to richest (right).
3) The Air Corrector -- The Air Corrector (AC) determines how much air is added to the mixture while the main jet and atomizer determine how much fuel is added, so the AC is a multiplier that effects the mixture of both the Atomizer and the Main Jet.
Lower numbers mean a smaller air hole, so less air and thus richer mixture. Most stock motors have a 160AC, which is too lean for any but the most mild tuning.
The Throttle Slide
Throttle slides have two cutouts, one on the back edge which sets the baseline opening for the idle flow and one underneath called the "cutaway" that allows extra air into the mixture when the opening is exposed to the carb's mouth.
Screenshot from FMP's Carb Slide video showing cutaways
Diagram showing how a cutout adds air
If both sides of the cutout are open, air will flow, leaning out the mixture. If they're not, it (mostly) won't. Selecting a slide is a matter of identifying a throttle range that needs to be leaned out and finding a slide whose opening is exposed through that portion of the range.
Basic Steps to Tune an SI Carb
1) Make sure the motor doesn't have any air leaks.
You will never jet your way around an air leak. Let me say that again: You will never jet your way around an air leak.
For details on how to do it, check out Getting Serious About Air Leaks.
Air leaks occur when a seal, gasket face, or other interface on the crankcase, cylinder, or head allows outside air in and mixture out. They tend to worsen over time and will cause lean conditions that only appear under certain running conditions, so jetting that seems good will eventually become too lean and cause a seize at the worst possible time.
Fortunately, you can pressure test a motor with a few dollars (pounds, euro's, peso's, rupii, hryvnia, beers, or whatever your local currency is) worth of parts and save yourself a few hundred of same by not having to replace your top end.
2) Pick An Air Corrector
This sounds arbitrary, but it's really not. From the factory, Vespas were tuned for fuel efficiency. That meant getting them to run reliably with the minimum amount of fuel possible and an Air Fuel Ratio (AFR) of 14.7, which provides maximum efficiency.
Note: The optimal AFR can be as low as 14.1, depending on exact fuel type. Don't @ me
Tuning a motor for maximum power, on the other hand, aims for an AFR of 12.7. So right out of the gate, our objectives are probably different than Piaggio intended for their motors.
If you're just adding a performance box exhaust, like a Polini Box, SIP Road series or BGM BigBox, you might get away with keeping the 160 AC and just upjetting the main and atomizer. But since you're ordering parts anyway, throw a 140 into your cart and save yourself the shipping if you need it later.
If you're putting on a 177 top end or adding a 60mm crank, just assume you need to swap to a 140 AC or, depending on how aggressively tuned, a 120. If you're doing builds that need a 120 AC, however, you should really only be reading this guide to provide me feedback and corrections.
3) Pick a Main Jet (MJ)
There are two really key things to understand about picking a main jet:
1) The maximum jet you can run in a stock SI carb is a 125 main jet without drilling the float bowl passage.
2) You need to start rich and work your way down.
Assuming you're tuning for a performance build, once you've drilled the float bowl passage, you can start on the actual jetting. You're going to want to have a full set of jets, from probably 135 down to 115.
For some reason, people always want to get cheap about buying jets. Just don't. People will spend a thousand bucks or more on go-fast parts (exhaust+crank+cylinder+carb) and then get cheap about the $4 bits of brass that are going to keep them from turning those parts into expensive garbage. Don't be that person.
4) Pick An Atomizer
The BE3 atomizer which most Vespas start with is also pretty lean. Highly tuned motors will go all the way to a BE4. It's also not uncommon to see a BE4 being used to cover up lean idle jetting at 1/4 throttle, either.
A guide for which atomizer to start with, which just occurred to me, is that from a BE3, go one jet richer for each performance part/task you're adding: Exhaust (+1 for a box, +2 for a tuned pipe), cylinder, crank, porting. So if you're doing the whole nine yards, you start with a BE4. If you're just upgrading your exhaust to a Polini box or SIP Road, a BE1 is a good starting point.
5) Pick an idle jet
Idle jets are marked with two numbers, e.g. "55/160." The two values are the size of the fuel and air openings, so the ratio of those two numbers is the real determinant of how rich or lean an idle jet is.
Divide the air value (160) by the fuel value (55) and you'll get a ratio, in this case 2.90. An idle jet with a lower value than 2.90 is going to be richer and a higher value will be leaner.
This chart is from the Calling all tech heads! Si Idle jet question(s)... thread, which gets into more detail.
I find that I usually wind up around a 52/140 idle jet, so I'll start with a 50/120 just to make sure. Again, you want to start rich and work your way down.
A too-lean idle jet will result in the motor not returning to idle after running, the "pip-pip-pip" sound, sometimes referred to as "hunting," where the motor seems to not quite want to settle back to idle. A seriously lean idle jet can refuse to settle down for thirty seconds or more. A healthy idle will go from WOT to within 100 RPM's of normal idling in 2-3 seconds.
A too-lean idle jet will also tend to "sag" when you start to accelerate. The motor won't bog down (that's too-rich), but it also will initially struggle to produce enough power to accelerate smoothly.
And on the too-rich side, the motor will struggle to idle smoothly and bog down, losing RPM's briefly when you first begin to accelerate. It will also tend to foul spark plugs on a regular basis if you're doing in-town riding with lots of starts and stops.
6) Tune it
The procedure is actually pretty simple:
6.0) Install a new spark plug. You don't want an old, crappy, half-fouled spark plug giving you bad results, because it will do things like make the motor seem rich when it's really the spark plug just sucking.
6.1) Start with the richest jet you have, put it in the motor, see if it'll rev on the stand. If it will, buy bigger jets. No hurry, this guide will still be here after they arrive.
6.2) Now, put the new biggest jet you have in the bike and take it out somewhere you can repeat the process on the road in 3rd gear. Walk the main jet down until it revs out cleanly in 3rd. The main jet is done. You might be tempted to keep going, and you can maybe go one jet smaller/leaner, but any more than that and you're starting to ask for trouble.
6.3) Tuning the atomizer is the same basic process as the main jet. Start rich, work your way down. If you have a 177 top end, start with a BE4. The Malossi 166 kit used to ship (maybe still does) with a BE4 and (I think) 125 Main Jet. No AC, though. Go figure.
If the BE4 is too lean, then you need to drop your Air Corrector (So 160 to 140; 140 to 120; etc.) and start the process over with the main jet, as it should now be too rich.
Once you're on the correct AC, test riding in 3rd gear by getting to the low end of the power curve, then dropping back to half throttle. It should continue to accelerate, albeit not as quickly, but also not stumble while it gets to a cruising speed.
6.4) Tune the idle
The idle jet is, at least for me, the most annoying jet to tune. It's done by balancing the idle jet size along with the mixture screw (the screw on the back of the carb). There are two thread pitches of mixture screw, what are referred to as "coarse" and "fine" pitch.
On coarse pitch screws, the default/center position is 1.5 turns out from when the screw bottoms in the carb (turn it in until it just barely stops moving. Don't force it).
On fine pitch screws, the default/center position of the mixture screw is 2.5 turns.
Turning the screw in will lean the idle mix, turning it out will richen it.
From left-to-right: Spaco fine pitch screw, Spaco coarse pitch screw, Dellorto coarse pitch screw. Notice they also all have different seats, so be sure that the screw matches the seat in the carb.
6.4.1) To start, set the mixture screw to its "center" position (1.5 or 2.5 turns out)
6.4.2) Take the bike out and get it warmed up. Really warmed up, like taken out and thrashed around for a few miles, not just ridden around the block. Figure ten minutes of riding at a minimum. If an excuse to go thrash your bike around for ten minutes doesn't sound like a good thing, you may want to find another hobby.
While you're riding, take note of whether the idle seems to be rich (bogging) or lean (hunting or sagging).
Once the bike is warmed up, swap up or down a jet depending on your initial observation. This isn't strictly necessary, but it'll potentially save you an iteration of the rest of the process.
6.4.3) Up the idle speed a few hundred RPM's. Most scooters idle between 900 and 1300 RPM's. If you have a tach, this is pretty easy. If not, then you want to turn it until it's "fast idling," so that you feel like it should settle back, but doesn't.
(The idle *speed* adjuster is the screw on top of the carb that protrudes through the air filter, btw.)
Adjust the mixture screw to the point where the idle is at its maximum speed. If it's more than 1/2 (coarse) or 3/4 (fine) turns away from center, then you need to swap to a richer (if it's turned out) or leaner (if it's turned in) idle jet, then re-center and repeat.
Once the jet and mixture screw are set, return the idle to its normal speed and blip the throttle. It should rev without hesitation, then drop back to a smooth idle in no more than 2-3 seconds with no hunting. Let it idle between blips for ten seconds or so to stabilize before making any final mixture screw adjustments.
5.5) Repeat to verify
Once all the components are set, the carb should be dialed in. Repeat the tests for each jet (3rd gear WOT for main, 1/2 throttle for atomizer, throttle blip for idle) and make sure everything sounds and feels good. If any part feels "off," re-tune that jet, then repeat the testing until they all feel good.
It may take a few iterations of the process to get everything dialed in, but eventually, you'll get there.
Once it's complete, make notes of your final setup along with observations along the way. I record every change, along with my observations of how the motor felt with every change in a table. It helps a lot and allows me to avoid any "going in circles" if I'm fighting with getting a particular part of the throttle range just right.
1/4 Throttle is the Bermuda Triangle of Tuning
If you look at the chart, 1/4 throttle has every variable in play at once, so any change made to the setup will impact it. Too lean and you're risking a seize when the motor is hot after a long WOT run and you come off the throttle. This is the infamous "off ramp seize." Too rich and it stumbles off the line, which is just irritating as can be.
Unfortunately, too many people only worry about being too rich at 1/4 throttle. Don't be that person. If you think stumbling off the line is embarrassing, try pushing your scoot up an off ramp some time and see how that feels.
1/4 Throttle is also one area where the carb slide can come into play when nothing else is working.
Install a Cylinder Head Temperature Gauge (CHT)
CHT's are not expensive ($30-60, depending on how fancy you get) and they can serve as both an Early Warning System and a Peace of Mind Provider while riding. After a while, you'll get to know what are normal temps for your bike, so if the temp is off, you know you need to figure out what's going on before something breaks and leaves you stranded.
Mark your throttle
Take tape, paint, a marker, or (my favorite) some nail polish and mark 0, 1/4, 1/2, 3/4, and WOT on your throttle. I put a dot on the throttle grip, then mark the headset at idle, then WOT, then split the middle for 1/2, then split again for 1/4 and 3/4. Visually verify that the throttle slide is actually at those positions, too.
If you have a lot of slop in your throttle cable, then you need to take up the slop to where the slide actually starts to move to mark idle. If you have excess throw, mark WOT at the throttle position where the slide first gets to fully open.
You can see how my throttle sits a little outside the marked range. That's me accounting for the slop in the cable.
Don't chase symptoms
By "chasing symptoms," I mean that most people tend to try to tune a carb by riding, deciding, "I don't like how it runs at ______________ throttle," so they arbitrarily try a different jet that may or may not actually impact that range of the throttle.
Don't be that person. Be the person who stands on the shoulders of giants, follows the process, and understands how the different jets impact the mixture. If you do that, even if it's your first time doing it, you should be able to get your carb fully tuned in an afternoon so you can get on to the fun part: taking your Vespa out in the world, confident it's going to bring you home again.
This whole post started with me finding the hand-drawn SI Jetting Effectiveness chart in a pile of papers on my desk, remembering I said I was going to draw it up into a more formal version, and somehow winding up down here.
I wasn't born knowing this, I learned by asking questions, researching, and trying to nail down a process-based approach rather than chasing symptoms.
This process is the accumulation of my experience over the years, along with reading the advice of many, many others on this forum, too numerous to mention, although Jack221 gets special mention as the official "Jet-eye Master" of Vespa tuning. Reading his guidance to others over the years not only taught me a ton, but also helped me learn how to think about the tuning problem effectively.
As always, comments, suggestions, improvements, criticism, and especially corrections, are all greatly appreciated.
SIP have an SI Carb Tuning Guide, too, that's pretty good and explains things differently, in case my explanations didn't work for you.
Stock jetting for most Vespa makes and models:
FreakMoped has produced a ton of good videos over the years on Vespa tuning, troubleshooting, comparing different tuning components, etc. Check out his YouTube channel for loads of good tuning videos.
And a shameless plug, I also publish videos to YouTube about all manner of Vespa stuff including tuning, bodywork, and whatever random maintenance task I need to perform that doesn't seem to have a good video explaining it.
Good luck, good tuning, and good riding!