OP
Sun, 17 Jul 2022 14:18:48 +0000

Lucky
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 08 Stella (for now)
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Lucky
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 08 Stella (for now)
Joined: Thu, 16 Jun 2011 14:59:35 +0000
Posts: 8434
Location: Nashville
Sun, 17 Jul 2022 14:18:48 +0000 quote
Please Note: This thread is for discussing the content of the SI Carb Tuning Wiki Page.

If you have questions regarding your own setup or carb tuning, please start your own thread!

It benefits everyone, because it will allow you (and the rest of us) to keep the conversation cogent and focused on your problem, meaning better guidance, plus provide you with a log of your tuning steps and process.

Tl;DR: Don't use this thread for scenario-specific questions; start your own thread. It's better for everyone.



Updates to this article are being made over on the Wiki page: Wiki [D] Guide: SI Jetting Chart + How to Tune an SI Carb, so look there for the latest & greatest version!
Official discussion thread for the wiki page: Wiki [D] Guide: SI Jetting Chart + How to Tune an SI Carb


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.



There are, for our purposes, only four jets and one adjuster screw in play in an SI carb:
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.



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.

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.

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.

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 Dellorto carbs, which have a coarse pitch screw, 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 Spaco carbs, which have a fine pitch screw, 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.

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.

Final thoughts:

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.

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.

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.

Epilogue
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.

Also, 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.

Good luck, good tuning, and good riding![/b]




Marked throttle. It doesn't need to be fancy. It does need to be accurate.



Last edited by chandlerman on Wed, 13 Sep 2023 18:39:51 +0000; edited 2 times
Sun, 17 Jul 2022 16:32:31 +0000

Hooked
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Hooked
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Sun, 17 Jul 2022 16:32:31 +0000 quote
Fantastic effort that is most appreciated. Thanks!

I've got mine running with your suggested setup from another thread and it's doing well. Thanks for that too.

Much to absorb here.
Sun, 17 Jul 2022 19:02:08 +0000

Molto Verboso
1958 Allstate 177VMC, 1962 Allstate, Yamaha Vino 70cc
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Molto Verboso
1958 Allstate 177VMC, 1962 Allstate, Yamaha Vino 70cc
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Sun, 17 Jul 2022 19:02:08 +0000 quote
Top notch. Thank you so much!
Mon, 18 Jul 2022 00:00:35 +0000

Molto Verboso
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Molto Verboso
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Mon, 18 Jul 2022 00:00:35 +0000 quote
Just wow, C'man.
Clear to see that was a LOT of work to put together.

Such delicate creatures, but solid when set up.

Can't have too many brass bits!
Even the no-way-in-hell-will-I-ever-use-this-one eventually finds a place somewhere, if even to rule it out.

Thinking about playing with a few of the atomizers I have laying around and a set of micro drill bits, seeing as they were made for a specific model's delicate interplay. It's not a tapered needle.
They were made rich somewhere and lean somewhere else to jet a specific bike at that time, and not for later tuning.

Glad you didn't bring-up slides...yet(?)



OP
Mon, 18 Jul 2022 00:35:58 +0000

Lucky
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 08 Stella (for now)
Joined: Thu, 16 Jun 2011 14:59:35 +0000
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Lucky
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 08 Stella (for now)
Joined: Thu, 16 Jun 2011 14:59:35 +0000
Posts: 8434
Location: Nashville
Mon, 18 Jul 2022 00:35:58 +0000 quote
Grumpnut wrote:
Fantastic effort that is most appreciated. Thanks!

I've got mine running with your suggested setup from another thread and it's doing well. Thanks for that too.

Much to absorb here.
Thanks! I was actually thinking partly about my post in your thread the other day and the fact that there should be a guide to this. Lots has been written about how to set up side draft carbs, but not much on SI's, at least not in one place.
Ray8 wrote:
Just wow, C'man.
Clear to see that was a LOT of work to put together.
It wasn't too bad. Honestly, it was an accident. I started out just intending to share the cleaned up version of the jetting effect chart. Then, I got into a flow, a couple hours and a pot of coffee later, it was done. Felt like maybe fifteen minutes, but the clock said otherwise.
Ray8 wrote:
Thinking about playing with a few of the atomizers I have laying around and a set of micro drill bits, seeing as they were made for a specific model's delicate interplay. It's not a tapered needle.
They were made rich somewhere and lean somewhere else to jet a specific bike at that time, and not for later tuning.
Ray8 wrote:
Glad you didn't bring-up slides...yet(?)
I skipped slides for a few reasons. They're so seldom needed, and honestly, I assume anyone who's at that level of delicacy with their tuning doesn't need my guide. Also, I'm not sure I have enough skills or experience with them to properly explain how to select the correct slide. I can add a little bit of the theory, though.

I'm also going to do some videos to take people through the jetting process, too, because it's hard to really describe rich vs. lean, for example. I'm going to need to splurge for a newer GoPro for that, though, to really do it right.
Mon, 18 Jul 2022 06:44:05 +0000

Banned
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Banned
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Mon, 18 Jul 2022 06:44:05 +0000 quote
I discovered today that the exact same stack from dellorto and SIP have drastically different results.

Have not pinned down what part is different, suspect it's the main, but the 118 dellorto main seemed to be richer than a 122 from SIP.

So stick with one make, don't mix them
Mon, 18 Jul 2022 07:13:29 +0000

Style Maven
'74 50s x3 '78 P200 E '84 Cosa '91 PK50XL2 - & - '58 AllState '68 Sprint '66(?) Super125 and '72 DanMotor Super150
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Style Maven
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Mon, 18 Jul 2022 07:13:29 +0000 quote
Very fine work, nicely polished! Let me add this here, someplace to start out for those totally lost.

Mon, 18 Jul 2022 08:34:14 +0000

Hooked
125 GTR 1977, 50S 1975, GTS 125 Supertech 2021
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Hooked
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Mon, 18 Jul 2022 08:34:14 +0000 quote
Great post! lots of useful information all put together in one place
Mon, 18 Jul 2022 09:32:31 +0000

Addicted
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Mon, 18 Jul 2022 09:32:31 +0000 quote
That's some excellent work there @chandlerman. Despite having mucked around with these carbs for quite a while it cleared up a few things in my head. Many thanks!

Wish I had that guide 10 years ago!
Mon, 18 Jul 2022 14:26:00 +0000

Veni, Vidi, Posti
'15 GTS300, '86 PX125EFL, '66 VBB, '01 ET4
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Veni, Vidi, Posti
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Mon, 18 Jul 2022 14:26:00 +0000 quote
seems like this should end up in NSM wiki....

agreed we need someone to fill in the gaps on slides. they do matter, especially at small throttles. there are a few variants out there. (varying depths of groove on the back of it). sounds like a job for Jack .

also would be nice to see - impact of filter (drilled vs not vs venturi) and which components of the stack to evaluate when moving between them.
OP
Mon, 18 Jul 2022 15:25:29 +0000

Lucky
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 08 Stella (for now)
Joined: Thu, 16 Jun 2011 14:59:35 +0000
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Lucky
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Joined: Thu, 16 Jun 2011 14:59:35 +0000
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Mon, 18 Jul 2022 15:25:29 +0000 quote
sdjohn wrote:
seems like this should end up in NSM wiki....

agreed we need someone to fill in the gaps on slides. they do matter, especially at small throttles. there are a few variants out there. (varying depths of groove on the back of it). sounds like a job for Jack .

also would be nice to see - impact of filter (drilled vs not vs venturi) and which components of the stack to evaluate when moving between them.
I'm in the process of adding it to the wiki now. It's not quite a cut n' paste, though, but it'll be over there later today.

I have testing the effect of venturi's on my to-do list, but just haven't gotten there yet. And drilled vs. undrilled is a little harder, because that's going to vary by setup and may even vary by filter, unless someone has a filter they want to drill to do the before & after measures.
Mon, 18 Jul 2022 15:40:37 +0000

Veni, Vidi, Posti
'15 GTS300, '86 PX125EFL, '66 VBB, '01 ET4
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Veni, Vidi, Posti
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Mon, 18 Jul 2022 15:40:37 +0000 quote
chandlerman wrote:
I'm in the process of adding it to the wiki now. It's not quite a cut n' paste, though, but it'll be over there later today.

I have testing the effect of venturi's on my to-do list, but just haven't gotten there yet. And drilled vs. undrilled is a little harder, because that's going to vary by setup and may even vary by filter, unless someone has a filter they want to drill to do the before & after measures.
Yeah I don't think we have to get specific, but something along the lines of if you plop a venturi on top you'd better think about adding x to your main jet and (maybe move the AC down 1 step?)

It's a good start, what you've got there. solid work.
OP
Mon, 18 Jul 2022 15:57:21 +0000

Lucky
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 08 Stella (for now)
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Lucky
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Joined: Thu, 16 Jun 2011 14:59:35 +0000
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Mon, 18 Jul 2022 15:57:21 +0000 quote
sdjohn wrote:
Yeah I don't think we have to get specific, but something along the lines of if you plop a venturi on top you'd better think about adding x to your main jet and (maybe move the AC down 1 step?)

It's a good start, what you've got there. solid work.
My venturi plan is to put the AFR on, get a baseline, then add the venturi, see how much it changed, re-jet back to baseline and then roll. I'm also going to do gsf dynos to see how much of a difference it really makes.
Mon, 18 Jul 2022 15:59:47 +0000

Not So Moderator
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Mon, 18 Jul 2022 15:59:47 +0000 quote
Wikitastic.
Mon, 18 Jul 2022 16:17:23 +0000

Veni, Vidi, Posti
'15 GTS300, '86 PX125EFL, '66 VBB, '01 ET4
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Veni, Vidi, Posti
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Mon, 18 Jul 2022 16:17:23 +0000 quote
chandlerman you're going to earn some toaster or shit for your profile
Mon, 18 Jul 2022 18:27:06 +0000

Hooked
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Mon, 18 Jul 2022 18:27:06 +0000 quote
Just added this to my favorites in the forum and also bookmarked it and printed it out in color to stick in my Red book.

Thank you so much!!!

H
OP
Mon, 18 Jul 2022 22:22:50 +0000

Lucky
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Lucky
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Mon, 18 Jul 2022 22:22:50 +0000 quote
Wiki page: Wiki [D] Guide: SI Jetting Chart + How to Tune an SI Carb
Official discussion thread for the wiki page: Wiki [D] Guide: SI Jetting Chart + How to Tune an SI Carb

I'm adding some more images as I convert this over to the Wiki, so there'll be some seemingly random noise because there seems to be no way to add images directly to the wiki, but I can link them from elsewhere on MV, so that'll be in this thread, along with any other discussion, suggestions, etc.








Slides. Shameless stolen from FMP's video.



OP
Mon, 18 Jul 2022 22:27:31 +0000

Lucky
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Lucky
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 08 Stella (for now)
Joined: Thu, 16 Jun 2011 14:59:35 +0000
Posts: 8434
Location: Nashville
Mon, 18 Jul 2022 22:27:31 +0000 quote
JimVanMorrissey wrote:
That's some excellent work there @chandlerman. Despite having mucked around with these carbs for quite a while it cleared up a few things in my head. Many thanks!

Wish I had that guide 10 years ago!
Thanks!

I wish I'd had it ten years ago, too, which is why once the words started flowing, I decided to just go with it.
Mon, 18 Jul 2022 22:56:56 +0000

Ossessionato
2007 Stella 225
Joined: Sat, 02 Nov 2019 20:44:07 +0000
Posts: 3507
Location: Rochester, Minnesota
 
Ossessionato
2007 Stella 225
Joined: Sat, 02 Nov 2019 20:44:07 +0000
Posts: 3507
Location: Rochester, Minnesota
Mon, 18 Jul 2022 22:56:56 +0000 quote
oopsclunkthud wrote:
I discovered today that the exact same stack from dellorto and SIP have drastically different results.

Have not pinned down what part is different, suspect it's the main, but the 118 dellorto main seemed to be richer than a 122 from SIP.

So stick with one make, don't mix them
https://www.sip-scootershop.com/en/product/jet-si-sip-performance-114_40261140
Mon, 18 Jul 2022 23:13:31 +0000

Style Maven
'74 50s x3 '78 P200 E '84 Cosa '91 PK50XL2 - & - '58 AllState '68 Sprint '66(?) Super125 and '72 DanMotor Super150
Joined: Tue, 23 Nov 2010 02:37:37 +0000
Posts: 9351
Location: seattle/athens
 
Style Maven
'74 50s x3 '78 P200 E '84 Cosa '91 PK50XL2 - & - '58 AllState '68 Sprint '66(?) Super125 and '72 DanMotor Super150
Joined: Tue, 23 Nov 2010 02:37:37 +0000
Posts: 9351
Location: seattle/athens
Mon, 18 Jul 2022 23:13:31 +0000 quote
Or bite the bullet and just get this set of ten:
Quote:
Vespa Carburetor Si Main Jet Set Type 4576 - 115-138 - BGM
$ 19.99
https://scootersoriginali.com/products/vespa-carburetor-si-main-jet-set-type-4576-115-138-bgm
Mon, 18 Jul 2022 23:16:04 +0000

Veni, Vidi, Posti
'15 GTS300, '86 PX125EFL, '66 VBB, '01 ET4
Joined: Fri, 05 Apr 2013 02:54:23 +0000
Posts: 7737
Location: San Diego, CA
 
Veni, Vidi, Posti
'15 GTS300, '86 PX125EFL, '66 VBB, '01 ET4
Joined: Fri, 05 Apr 2013 02:54:23 +0000
Posts: 7737
Location: San Diego, CA
Mon, 18 Jul 2022 23:16:04 +0000 quote
+1 on the BGM jet kits
Tue, 19 Jul 2022 13:15:11 +0000

Hooked
1959 VBA ,166, original 8's. 2005 Stella 177
Joined: Sat, 10 Apr 2021 19:33:23 +0000
Posts: 263
Location: Winder GA
 
Hooked
1959 VBA ,166, original 8's. 2005 Stella 177
Joined: Sat, 10 Apr 2021 19:33:23 +0000
Posts: 263
Location: Winder GA
Tue, 19 Jul 2022 13:15:11 +0000 quote
Thanks for the work on this.
Tue, 19 Jul 2022 16:22:48 +0000

Hooked
125 GTR 1977, 50S 1975, GTS 125 Supertech 2021
Joined: Mon, 04 Aug 2014 16:38:57 +0000
Posts: 277
Location: Madeira Island
 
Hooked
125 GTR 1977, 50S 1975, GTS 125 Supertech 2021
Joined: Mon, 04 Aug 2014 16:38:57 +0000
Posts: 277
Location: Madeira Island
Tue, 19 Jul 2022 16:22:48 +0000 quote
sdjohn wrote:
+1 on the BGM jet kits
Plenty to play with. Just a note, up to 120 they are super accurate but on bigger sizes they are a bit smaller than the rated number, like -0.02 mm

This was my initial 140 that is actually more like a 146, it was an original 98 which was enlarged with a 1.4mm drill ... so not much unexpected



Wed, 20 Jul 2022 13:44:12 +0000

Ossessionato
Vespas 1964 GS160, 1965 SS180, 1977 V9A1T, 1983 PX150E
Joined: Sun, 17 Apr 2011 00:34:49 +0000
Posts: 2468
Location: Siam
 
Ossessionato
Vespas 1964 GS160, 1965 SS180, 1977 V9A1T, 1983 PX150E
Joined: Sun, 17 Apr 2011 00:34:49 +0000
Posts: 2468
Location: Siam
Wed, 20 Jul 2022 13:44:12 +0000 quote
V oodoo wrote:
Very fine work, nicely polished! Let me add this here, someplace to start out for those totally lost.


It should be noted that the GS160 Mark II runs a hell of a lot better with a 115 main jet instead of the 108 that is published. There is a thread on here from Pilaf who explained it well. My GS now is a joy to ride with the 115 whereas it used to be a burden with the old 108.
⬆️    About 1y elapsed between posts    ⬇️
Fri, 28 Jul 2023 00:04:51 +0000

Hooked
1965 Allstate, 1979 Rally 200
Joined: Thu, 16 Feb 2012 16:24:57 +0000
Posts: 136
Location: Philadelphia, Pa
 
Hooked
1965 Allstate, 1979 Rally 200
Joined: Thu, 16 Feb 2012 16:24:57 +0000
Posts: 136
Location: Philadelphia, Pa
Fri, 28 Jul 2023 00:04:51 +0000 quote
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.





I know this is an old thread but it is very helpful and thorough. Question though... are you saying the numbers on the atomizers are not in corrolation to the size of them? This chart shows the numbers being completely random? A BE3 is larger then a BE4?
OP
Fri, 28 Jul 2023 00:14:09 +0000

Lucky
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 08 Stella (for now)
Joined: Thu, 16 Jun 2011 14:59:35 +0000
Posts: 8434
Location: Nashville
 
Lucky
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 08 Stella (for now)
Joined: Thu, 16 Jun 2011 14:59:35 +0000
Posts: 8434
Location: Nashville
Fri, 28 Jul 2023 00:14:09 +0000 quote
dsmith181 wrote:
I know this is an old thread but it is very helpful and thorough. Question though... are you saying the numbers on the atomizers are not in corrolation to the size of them? This chart shows the numbers being completely random? A BE3 is larger then a BE4?
That's correct. The numbering is the order in which they were made, not a scale of rich-to-lean or vice versa.
Fri, 28 Jul 2023 01:01:16 +0000

bodgemaster
63 GL, 76 Super (x2), 74 Primavera (x2), 79 P200, 06 Fly 150
Joined: Thu, 26 Sep 2013 22:01:08 +0000
Posts: 6942
Location: So Cal
 
bodgemaster
63 GL, 76 Super (x2), 74 Primavera (x2), 79 P200, 06 Fly 150
Joined: Thu, 26 Sep 2013 22:01:08 +0000
Posts: 6942
Location: So Cal
Fri, 28 Jul 2023 01:01:16 +0000 quote
The numbers mean nothing. They could just as easily have named them Luigi, Guiseppi, Angelo, Vito, Mario, etc.

Which IMO would've been less confusing and more fun.
Fri, 28 Jul 2023 14:16:50 +0000

Hooked
1965 Allstate, 1979 Rally 200
Joined: Thu, 16 Feb 2012 16:24:57 +0000
Posts: 136
Location: Philadelphia, Pa
 
Hooked
1965 Allstate, 1979 Rally 200
Joined: Thu, 16 Feb 2012 16:24:57 +0000
Posts: 136
Location: Philadelphia, Pa
Fri, 28 Jul 2023 14:16:50 +0000 quote
chandlerman wrote:
That's correct. The numbering is the order in which they were made, not a scale of rich-to-lean or vice versa.
Got it. So the goal is to drive everyone nuts trying to make these things run right.
OP
Fri, 28 Jul 2023 14:31:30 +0000

Lucky
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 08 Stella (for now)
Joined: Thu, 16 Jun 2011 14:59:35 +0000
Posts: 8434
Location: Nashville
 
Lucky
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 08 Stella (for now)
Joined: Thu, 16 Jun 2011 14:59:35 +0000
Posts: 8434
Location: Nashville
Fri, 28 Jul 2023 14:31:30 +0000 quote
dsmith181 wrote:
Got it. So the goal is to drive everyone nuts trying to make these things run right.
If not the goal, then definitely the outcome.

I think there's a reason that pretty much the entire rest of the carb world uses side draft rather than si carbs.
Fri, 28 Jul 2023 14:45:58 +0000

Banned
3:5
Joined: Fri, 12 Jan 2007 02:38:46 +0000
Posts: 8740
Location: San Francisco
 
Banned
3:5
Joined: Fri, 12 Jan 2007 02:38:46 +0000
Posts: 8740
Location: San Francisco
Fri, 28 Jul 2023 14:45:58 +0000 quote
chandlerman wrote:
...I think there's a reason that pretty much the entire rest of the carb world uses side draft...
um...



OP
Fri, 28 Jul 2023 14:52:49 +0000

Lucky
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 08 Stella (for now)
Joined: Thu, 16 Jun 2011 14:59:35 +0000
Posts: 8434
Location: Nashville
 
Lucky
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 08 Stella (for now)
Joined: Thu, 16 Jun 2011 14:59:35 +0000
Posts: 8434
Location: Nashville
Fri, 28 Jul 2023 14:52:49 +0000 quote
oopsclunkthud wrote:
um...


I stand corrected.
Fri, 28 Jul 2023 14:56:37 +0000

Banned
3:5
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Location: San Francisco
 
Banned
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Location: San Francisco
Fri, 28 Jul 2023 14:56:37 +0000 quote
I still really want to try a down draft BN on a vespa. It's been done before back in late 80's and worked well with an RZ cylinder.
Fri, 28 Jul 2023 16:58:39 +0000

Jet Eye Master
PX221 MHR, O tuned PX200, PX181 Quattrini and some motorbikes
Joined: Thu, 15 Jun 2017 05:16:54 +0000
Posts: 4112
Location: London UK
 
Jet Eye Master
PX221 MHR, O tuned PX200, PX181 Quattrini and some motorbikes
Joined: Thu, 15 Jun 2017 05:16:54 +0000
Posts: 4112
Location: London UK
Fri, 28 Jul 2023 16:58:39 +0000 quote
SI carbs were chosen for their simplicity and economy......but only when left bone stock.
Wed, 09 Aug 2023 21:35:46 +0000

Hooked
1965 Allstate, 1979 Rally 200
Joined: Thu, 16 Feb 2012 16:24:57 +0000
Posts: 136
Location: Philadelphia, Pa
 
Hooked
1965 Allstate, 1979 Rally 200
Joined: Thu, 16 Feb 2012 16:24:57 +0000
Posts: 136
Location: Philadelphia, Pa
Wed, 09 Aug 2023 21:35:46 +0000 quote
"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."

So I just got a 24/24 e carb for my 79 rally 210 malossi road 2.0 and I have a 28 main in there. It's starving at Wot. I haven't drilled out the float bowl passage. According to this there's no point in a bigger main without drilling out first?
Wed, 09 Aug 2023 22:19:51 +0000

Molto Verboso
Joined: Sun, 29 Nov 2020 21:32:23 +0000
Posts: 1552
Location: Los Angeles
 
Molto Verboso
Joined: Sun, 29 Nov 2020 21:32:23 +0000
Posts: 1552
Location: Los Angeles
Wed, 09 Aug 2023 22:19:51 +0000 quote
dsmith181 wrote:
"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."

So I just got a 24/24 e carb for my 79 rally 210 malossi road 2.0 and I have a 28 main in there. It's starving at Wot. I haven't drilled out the float bowl passage. According to this there's no point in a bigger main without drilling out first?
Get this bit.
https://www.amazon.com/gp/product/B0062I8XAI/ref=ox_sc_saved_title_1?smid=A2Q1LRYTXHYQ2K&psc=1

Tape it 10mm from the tip and don't go deeper.
Remove the tape and confirm you can see the end of the bit while looking down the main jet bore with a flashlight.
Blow the shit out of every channel with compressed air.

This is a big bit (2.7mm), but it acts like when you use a garden hose to wash a patio.
Bigger hose, smaller nozzle gets the job done with less water (fuel) than a narrow hose, larger nozzle.

Regarding your atomizer confusion, the good thing is most any setup at this level of tune will work with a BE3.
Aside from the BE5, BE2, and maybe the BE4, the others can be disregarded.
Good luck!
Wed, 09 Aug 2023 23:47:09 +0000

Hooked
1965 Allstate, 1979 Rally 200
Joined: Thu, 16 Feb 2012 16:24:57 +0000
Posts: 136
Location: Philadelphia, Pa
 
Hooked
1965 Allstate, 1979 Rally 200
Joined: Thu, 16 Feb 2012 16:24:57 +0000
Posts: 136
Location: Philadelphia, Pa
Wed, 09 Aug 2023 23:47:09 +0000 quote
Alright thanks.

I'll do that.
The funny thing is I had a 26/26 (t5 so the shirt jets were hard to find too) on there and had so much trouble tuning that I got a 24/24 thinking it would be easier. Now I'll drilling a hole in it making it more like the 26
Thu, 10 Aug 2023 00:37:40 +0000

Molto Verboso
1958 Allstate 177VMC, 1962 Allstate, Yamaha Vino 70cc
Joined: Sun, 23 Aug 2020 21:02:46 +0000
Posts: 1608
Location: Philadelphia
 
Molto Verboso
1958 Allstate 177VMC, 1962 Allstate, Yamaha Vino 70cc
Joined: Sun, 23 Aug 2020 21:02:46 +0000
Posts: 1608
Location: Philadelphia
Thu, 10 Aug 2023 00:37:40 +0000 quote
dsmith181 wrote:
Alright thanks.

I'll do that.
The funny thing is I had a 26/26 (t5 so the shirt jets were hard to find too) on there and had so much trouble tuning that I got a 24/24 thinking it would be easier. Now I'll drilling a hole in it making it more like the 26
It won't make it like a 26. It'll just allow the engine to get the proper amount of fuel is all. Stock carbs have a bottleneck above 125mj, So a 140 acts the same as a 126. So to speak. I think. Maybe.
OP
Thu, 10 Aug 2023 03:09:07 +0000

Lucky
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 08 Stella (for now)
Joined: Thu, 16 Jun 2011 14:59:35 +0000
Posts: 8434
Location: Nashville
 
Lucky
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 08 Stella (for now)
Joined: Thu, 16 Jun 2011 14:59:35 +0000
Posts: 8434
Location: Nashville
Thu, 10 Aug 2023 03:09:07 +0000 quote
FridayMatinee wrote:
It won't make it like a 26. It'll just allow the engine to get the proper amount of fuel is all. Stock carbs have a bottleneck above 125mj, So a 140 acts the same as a 126. So to speak. I think. Maybe.
Pretty much. Given that the pressure is constant since it's defined the "head height" of the float bowl, you have to increase the diameter of the passage to increase flow.

You could potentially put a taller float bowl on the carb, too, but I don't know how much of a difference a few millimeters would make. I have one, and it also uses a larger needle with a larger opening into the float bowl from the fuel line, but have never measured it, so don't know what difference it is making.
Thu, 10 Aug 2023 22:30:49 +0000

Hooked
1965 Allstate, 1979 Rally 200
Joined: Thu, 16 Feb 2012 16:24:57 +0000
Posts: 136
Location: Philadelphia, Pa
 
Hooked
1965 Allstate, 1979 Rally 200
Joined: Thu, 16 Feb 2012 16:24:57 +0000
Posts: 136
Location: Philadelphia, Pa
Thu, 10 Aug 2023 22:30:49 +0000 quote
Ok. Did this. Just to confirm. Only drill 10mm and that's it. It creates a wide hole and then a small one.
The bit shouldn't go all the way through
Ray8 wrote:
Get this bit.
https://www.amazon.com/gp/product/B0062I8XAI/ref=ox_sc_saved_title_1?smid=A2Q1LRYTXHYQ2K&psc=1

Tape it 10mm from the tip and don't go deeper.
Remove the tape and confirm you can see the end of the bit while looking down the main jet bore with a flashlight.
Blow the shit out of every channel with compressed air.

This is a big bit (2.7mm), but it acts like when you use a garden hose to wash a patio.
Bigger hose, smaller nozzle gets the job done with less water (fuel) than a narrow hose, larger nozzle.





OP
Thu, 10 Aug 2023 22:46:27 +0000

Lucky
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 08 Stella (for now)
Joined: Thu, 16 Jun 2011 14:59:35 +0000
Posts: 8434
Location: Nashville
 
Lucky
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 08 Stella (for now)
Joined: Thu, 16 Jun 2011 14:59:35 +0000
Posts: 8434
Location: Nashville
Thu, 10 Aug 2023 22:46:27 +0000 quote
dsmith181 wrote:
Ok. Did this. Just to confirm. Only drill 10mm and that's it. It creates a wide hole and then a small one.
The bit shouldn't go all the way through
You want to drill the entire passage, but not all the way through and out the bottom of the carb. If you've done it correctly, you'll be able to see the to of the drill bit when you look down into the jet stack hole
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