Modern Vespa

Jets that have the same AF ratio (e.g. a 42/140 and a 48/160) flow the same amount of total mixture at idle.

True or false?

False. Same ratio. 48/160 flows more total volume of that mixture...?

well if the ratios are the same...I would guess that the ratios might be different at different throttle positions. theoretically the ratios might be the same at WOT.
just a thought...scott

Ok. So that's one false and one not sure.

Anybody else?

There is also 120/36 with the same ratio.

My guess is that the statement is true.

Interested to know.

I think Scott has the right answer

Hello SoCalGuy.
You're trying to make sense of the jet ratio. Jack221 doesn't give a definite answer and He's an SI Carb Grand Wizard

Yes, Dellorto was playing some cruel joke on us with these carbs.

FWIW, my guess is "false" ... logically, the jet with the larger fuel opening should allow slightly more fuel to flow at idle. But I think both can deliver the same flow by adjusting the mixture screw. Off idle is where they begin to perform differently due to the different sized air holes.

I would think that the A/F ratio of the pilot circuit would remain the same, but don't forget that this circuit overlaps with the main circuit and the total A/F fuel ratio delivered will depend on the amount of that overlap. It's also my understanding that the pilot jet with the smaller air corrector component would have less overlap and vice versa, so you'd have to factor all that in.

The SI carb appears to be a dumbed down version of a basic Weber carb which this short article helps to understand. (The original link appears to have been taken down, but I found a copy on the Wayback Machine here.)

https://web.archive.org/web/20180807225708/http://www.datsun2000.com/tech/weber_tuning_methodology.htm

So true or false Jim?

Note that on a Weber, the "mix screw" controls the volume of both air and fuel. On an SI it only controls fuel.

I say false also, for the same reasons you stated SoCalGuy. The inlet is bigger so at idle, the AF mixture will be different.

At idle, the total mixture delivered is going to depend on the where the mix screw is since it's a volume screw (not an air screw) for the pilot circuit and the main circuit isn't in play yet, so I think the answer is true, but only at idle. Once off idle, things get complicated and it becomes the frustrating black art we're all familiar with, more or less.

Hmmm, this is a good question.

Building on what Scott said, what pdxjim says I think is also correct (apart from the pilot and main circuit bit).

All the info on the Webber emulsifier tubes, applies to the SI carb exactly. Same concept. Fuel is delivered on a curve based on emulsified mixture reacting to the change in vacuum pressure.

Some already know this but for clarity; the SI mixture screw ONLY works while the throttle is closed and until just slightly open. After about 2mm of throttle opening the slide uncovers the 'progression hole' which bypasses the mixture screw adjustment entirely.

Another relevant difference with the SI carb is that as there is no mechanical valve, like a slide needle to regulate the main jet flow. This means that the main jet stack is always online. Even at tickover the main jet is flowing something. So if the main jet is way off the pilot jet will also be way off. Not sure this is now less confusing but there you go.

Appreciate that info Jack.

The reason I brought this up is because I just switched from a 48/160 (3.33) to the next "richer" 40/130 (3.25) to address a hanging idle.

The 40/130 is much better, nice and snappy, idle doesn't hang, temps are good, but it got me wondering if the smaller fuel opening might be a problem...

Btw, I also tried a 50/160 (3.125) - too boggy off the line.

Whats the set up of this scooter?

Even a stock 125 will run bigger than a 40. I appreciate it runs better than before but doesn't make it right. The mixture screw will be more open than it should be to hide the weak area. 40/130 on a PX125 with a Spaco carb probably is 3.5 turns or more. This would then not give a smooth curve.

A weak pilot circuit can just be a weak main stack.

Did you try a 45/140?

^^^This makes sense to me because the overall volume from the circuit is reduced, but isn't a 45/140 (3.11) quite a bit richer than 48/160 (3.33)?

Also, aside from the enrichening effect, doesn't the smaller A/C mean less overlap with the main circuit?

I'm wanting to understand your reasoning here, Jack.

Here's a little chart I lifted from someone else once to help me keep this sorted. I added additional part numbers as I discovered them.

I'd like to hear Jack's reasoning too. I'm still not sure he's given us an answer to the quiz question.

I guess I'm trying to better understand the importance of the numerator - the fuel nozzle size - in the SI idle jet equation XX/YYY, as opposed to the overall ratio. Is it the mixture screw setting alone that determines whether you have an appropriate fuel opening? Too many turns out, the opening is too small. Too many turns in, the opening is too large?

I think the charts like the one above that list SI jets in order of "richness" and overlap are a little misleading. They tell us nothing about what is or isn't appropriate for a particular displacement engine. A 38/120 (3.16), for example, has a closer fuel-to-air ratio than a 50/160 (3.20), and is shown as technically "richer", but it flows a significantly smaller volume of total mixture. You might choose it to richen or weaken the idle on a P125, but is it appropriate for, say, a Rally 180?

It's not as straightforward as just looking at the AF equation. And another problem with the SI carb is that every SI pilot jet comes already married to a particular air opening, which affects fuel delivery beyond just idle.

First - waita minute - you didn't have the sure answer before you posted?
Second - love this post

On my 150, three port (I think) with slightly modified inlet/ SI 2020/ I experienced almost the exact same with idle jet as you.

40/130 = 3.25 and below just fail to provide any snappiness.
Dip below 3.0 in any manner and feels thick with bog.
48/160 = 3.33 made all the difference in first turn throttle crispness.
Feels right through the range now (with 160/BE3/100 main stack).

In my case - 42/140 would have delivered same 3.33 ratio - at idle - but never tried.
(or a 50/160 could have matched my 3.25 ration which was my next closest happy set up).

What are the motor/specs for yours?

This is the yellow soop. Three port, 177, GS piston, 20/20, big box, 160/BE4/116.

48/160 pilot was close but slow to return to idle after long pulls under load/uphill. Cleaned right up with 40/130. Temps well under 300F. Been on freeway a few times now and still haven't cracked 350F.

The only sure answer with SI carbs is Dellorto made a pile of dough selling jets.

Bingo! It's not as straightforward as just looking at the AF equation

The fundamental thing that is missing is flowrate. Even if the A/F ratio is the same between two jets the flow rate isn't. If your mixture screw doesn't end up at 2.5 turns (Spaco) when adjusted, then you have the wrong flowrate. A/F ratio might be spot on but who cares when its got a big boggy dead spot. This overlap thing is going to confuse. Although there is obviously overlap It really doesn't matter in the pilot jet decision. The main jet overlap with the pilot is a much bigger issue, especially with the more powerful set ups.

I don't know if this helps anyone or not, but I cleaned up and rearranged the table above a bit.

Sort the table into size of the jet and it will be more useful.

This table makes it seem like you have to stick to idle jets whose air size matches air corrector sizes.. ie 160 air corrector should only use idle jets with x/160. I've seen this mentioned before somewhere, but in my mind it seems to make more sense to use whichever idle jet works appropriately. Otherwise you are limiting yourself to large jumps of af mixtures.. there are many other sizes jets between 42/140 and 45/140, and if that means something between those two jumps of x/140 works appropriately, then you use it. Am I completely wrong and as Jack suggested the flow will be wrong if you use other jets with differing air sizes to the air corrector?

Swiss - As I read it - the numbers across the top are the second number of the idle jet - rather than an AC jet.
The AC jet, atomizer, and main are not shown/ unrelated to this chart.
Took me a minute to grasp it as well - of course - I might also be misreading...

:-)

Flow won't necessarily be "wrong" but it'll be different. And the difference in flow will affect other parts of the throttle, and adjustments will need to be made to the parts of the carb that control those other circuits.

I've come to the conclusion that if you're changing idle jets you should stick to the same air corrector if possible. Using the straw analogy, the idle jet and main jets sucking through the same size straw gives a more even and predictable flowrate. One change at a time.

For example, if you have a 50/160 and want richer go for a 55/160, not a 38/120. The 38/120 may theoretically be "richer", but you run the risk of cooking your top end coming off WOT to a seriously undersized idle jet.

I didn't say keep the same atomiser size or air corrector size. To get the right jet anything goes but if you're looking for a guideline that make this simpler with less damage, then this one will help.

Don't fit a pilot jet that has the small number over 5 points less than the stock one.


This will keep you safe from rolling off the throttle after a satisfying thrash and having an unscheduled pit stop.

I am jumping back in on this thread as its the most appropriate place for me to as my questions.

I'm riding a Stella with an un-ported BGM 177 on it. I finally have the majority of the jettiing dialed in with the following stack:

AC20/BE4/122
55/160 Idle Jet

I have great throttle response from 1/4 through to WOT but it still feels a little popcorn poppery in the first 1/4 of the throttle.

My temptation is to try a 45/120 which in theory is richer and I think would solve the poppery in the same way that moving from the BE3 to the BE4 richened things up and made it smoother.

But! There is a lot of information above which confuses me like "don't move the an idle jet where the lower number is 5 points less than stock." This makes sense if I look at the range of jets in the 160 column as going to a lower small number will richen things up, but changing to 45/120 should make me richer overall.

Hal

The 45/120 is theoretically a richer idle jet than a 55/160 because it has a higher fuel-to-air ratio. But the fuel passage is smaller, so the volume of fuel it furnishes will be less.

But I think you'll be safe. The mixture screw setting will tell you if it's useable for your engine. Too many turns out, too small. Too many turns in, too big.

* edited to correct mistakes and clarify - sorry for confusion

What would be 'too many turns'?

1.5 rotations from fully screwed in is the ideal position on an SI carb.

If you need to turn the mixture screw more than 1/2 a rotation away from that position to get a smooth idle with a fully warmed up engine and the idle speed set slightly higher than normal, the idle jet is too rich or too lean.

At normal idling speed, the engine should rev up immediately with a twist of the throttle then immediately fall back to the idling speed.

Great thread. This is exactly the info I was looking for…the joys of tamping down 1/4 throttle 4 stroking without setting up for a seize when rolling off the throttle.

Good info but note that the 1.5 turns out is for the original Dellorto SI carbs. For the more recent Spaco versions (which is what Stellas used) the target is 2.5 turns out.

Thanks for the info guys

I don't understand how the 45/120 is leaner. According to this chart which is above, the 45/120 gets less air and more fuel.



That is entirely why I'm looking at the 45/120 which has an AF ratio of 2.67 vs going to a 45/140 or something above 2.91.

Hal

Sorry for the confusion Hal. I edited my post above. Hopefully it makes more sense now.

The AF number of an idle jet doesn't tell even half the story. Its just used as an average way of comparing jets for the same static pressure. This AF number actually means very little, as the venturi pressue is not static across the range of pilot operation (unless its not running!). If you want a richer pilot mixture, use a jet with a bigger fuel number than the previous. And keep the air number the same.

There is no X.X turns-out target/rule on a modified bike.

Jetting for smooth acceleration and CHT response, fixed turns-out with a Spaco carb and a BE4/120 will be like chasing a greased pig, from my statistically significant experiences

You have to take it out hot and play with it!

At around 10:00 here:

Have no idea what he was saying but understood perfectly when he tossed the screw in disgust.

I was glad that I had a few days to ponder all of this before I started monkeying around. I thought a lot about SoCalGuys comments on the idle jet and I decided to keep the 55/160 for the time being as I'm only running a 177 and that idle jet is stock in a P200 so it should be plenty rich.

Instead I just went from a 122 to a 125 MJ and that completely woke everything up. It's a completely different bike now. There is still just a touch of pop-pop at low throttle but none of the surging feel that I now think is indicative of lean running. I may eventually move up to a 130MJ just to see if I can get over rich.

I'm at 2.5 turns out but now that I read the most recent posts I think the happy place is at about 2 turns. I'll post more when I test further but this degree of performance out of a bolt on jug and slug is remarkable.

Hal

true.

The video that defies auto-translate. The birds in his yard don't help, but even native Germans struggle with his SI videos.

"On a bone stock bike, yeah. X.X turns out ."

"The mixture screw on a tuned bike MUST be adjusted. I have bikes running anywhere from 1 to 5, or even 6.
Not doing so is just stupid."

Then he tosses the screw, which auto-translates as "Clapping."