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I have been reading everything I can find on jetting and there are some things I'm still trying to figure out.

Main jet stack and air correctors - It's possible to get the same fuel/air ratio using a different combination of air correctors and main jets. For example, using a 160 AC with a 120 main jet would give you the same air/fuel ratio as 140 AC with a 105 main jet. My first guess that the combination using the larger main jet would deliver a greater volume of fuel. Am I correct in my thinking? Are there other differences between these 2 combinations?

I was also looking at the same thing with idle jets. My understanding in this case is that a lower air number has less overlap with the main jet compared to a combination with a higher air number that yields the same mixture ratio. Is this also because there is less volume of fuel delivered?

Last thing is atomizers. It looks like they are used to balance out the mixture between low, mid and full throttle to to fine tune the throttle range.

Thanks for reading along.
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There are a few "simple" formulas for Dellorto PHBH style carbs...
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SubEtherBASS wrote:
There are a few "simple" formulas for Dellorto PHBH style carbs...
If you have a maths degree in algebra then fill your boots.
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ferriswolf wrote:
If you have a maths degree in algebra then fill your boots.
I'm still trying to understand the basics of jetting, so my question isn't as complicated as it sounds.

Take 2 AC and MJ combinations that yield the same mixture ratio, like:

AC 140/MJ 105 vs AC 160/ MJ 120

How would they be different in how the bike went down the road? Why would you choose one combo over the other?

And question about the mixture tube. They are often described in terms of rich vs lean, but my understanding is that they also balance out the mixture across the throttle range…for example leaning out low end and enriching full throttle. Just wondering if I'm correct in my understanding.

Thanks
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I don't think you can just go by the AC and MJ. Atomizer does the "balancing", and you have three areas to deal with: low throttle, mid, and 7/8-WOT. The atomizer will mix differently at different throttle positions.

Then there is the optimal temp of the engine at low/mid/high, and exhaust gas temp.

Also, what does the plug look like, and the wash on the piston crown?

So many factors that I don't have enough energy and effort to figure that all out. I really like to ride, so what did I do to understand this all?

I followed the experts advice, namely JetEye Jack, CM, Oops, Rob Hodge, Bald John, And various other experts * and advice from everyone's posts. And studying pictures.

Then I make up my own mind on what to try, after my pea brain bangs it around for awhile.

But now - I just follow JetEye Jacks advice.





* Please forgive me if I missed you in this list. You are appreciated!
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Thank you.
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But yes you're right about ac and mj differences , an ac120 will use a lot lower mj then say , having an ac140 because the ac120 is the richest air corrector . I've ended up jetting my scoots by getting initial advice for the ballpark setup versus the kit being used. Some advice not working out and some being right but advice given is hard unless you got the scoot in front of you or having experience of certain makes , eg polini , DR , malossi , all have unique characteristics from my experience and need various combos of ac, mj and mixer and often forgotten the pilot jet. I think your asking whether you can end up with same result by compensation the ac and mj, I think that's very unlikely across all throttle running conditions and atmospheric adjustments , altitude temperature etc
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ferriswolf wrote:
But yes you're right about ac and mj differences , an ac120 will use a lot lower mj then say , having an ac140 because the ac120 is the richest air corrector . I've ended up jetting my scoots by getting initial advice for the ballpark setup versus the kit being used. Some advice not working out and some being right but advice given is hard unless you got the scoot in front of you or having experience of certain makes , eg polini , DR , malossi , all have unique characteristics from my experience and need various combos of ac, mj and mixer and often forgotten the pilot jet. I think your asking whether you can end up with same result by compensation the ac and mj, I think that's very unlikely across all throttle running conditions and atmospheric adjustments , altitude temperature etc
I imagine there would be a difference. Just trying to understand it better, but it's one of those things that you end up learning by experience…getting it close enough to ride with caution, feeling how it rides, sounds, etc, making one change at a time and seeing how it goes. It takes some time to attune yourself, if that makes sense.
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orwell84 wrote:
I'm still trying to understand the basics of jetting, so my question isn't as complicated as it sounds.

Take 2 AC and MJ combinations that yield the same mixture ratio, like:

AC 140/MJ 105 vs AC 160/ MJ 120

How would they be different in how the bike went down the road? Why would you choose one combo over the other?

And question about the mixture tube. They are often described in terms of rich vs lean, but my understanding is that they also balance out the mixture across the throttle range…for example leaning out low end and enriching full throttle. Just wondering if I'm correct in my understanding.

Thanks
The dividing thing is just a guide, it doesn't quite work linear.
In your example, if the AC140 had a ~108, at WOT both jettings would be similar. However, the AC140 would get richer quicker. If both were tried in the same engine, they would need completely different atomisers.
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Jack221 wrote:
The dividing thing is just a guide, it doesn't quite work linear.
In your example, if the AC140 had a ~108, at WOT both jettings would be similar. However, the AC140 would get richer quicker. If both were tried in the same engine, they would need completely different atomisers.
That's kind of what I wanted to know and makes perfect sense. I'm definitely making progress. I read a lot on jetting that is now starting to make sense as I ride more and more. I read through again and it becomes even clearer.

Yesterday, I went from a 115 MJ to a 122. I had tried the 122 on one of my first attempts and thought it was too rich. I didn't know what I was hearing/feeling…

When I tried it yesterday, I realized it was actually much better. I had confused the 4 stroking at 1/4 throttle with a too rich MJ. It wasn't until I marked the throttle that it started to make sense. After that I tried a 125 MJ and it was obviously too rich as it spluttered throughout the throttle range. I couldn't tell the difference when I first started but now I can.

Last thing is setting the the idle and mixture screws. They have a much bigger contribution on a Vespa than on a 4 stroke car. I'm still getting this down.

Thanks for responding and for reading along with my ramblings.
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Its a dark art, and yes plenty of experts on here which is really helpful. You will get contrasting advice and you will learn from doing. Its taken me years to fully understand it and its only by spending a lot of time trying that you get there with each scoot. You will know how your scoot performs, the sound of it, the response of it, the plug colour, the bogging, the lean dry sounding engine etc. Every single one is different. If you post your current engine setup, exhaust, carb etc, you will get a bullpark config from guys on here and from there you can then experiment. Important thing is that you get a richer config to start out with because at least your engine isnt going to go bang, its when you start too lean that the issues start. Little things make changes too, last winter in the UK temperature dropped to 1c and the characteristics of my engine went so lean, so upped a few jets and perfect. Jets used to be cheap but seem rather expensive now for what they are. Dont buy cheap jet kits either, they have been found to be vastly different from what is stated. I seem to remember that SIP jets can be bigger than written too, some posts on MV sometime back saying that the case. I stick to official Del Orto Jets which are more expensive but at least i know they are correct to best of my knowledge.
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