Modern Vespa

Yeah. Definitely lean now. Something missed with the bigger main jets. I wonder if it was ever rich enough. Start at 140mj this time. Be sure it's absolutely pig rich. The key word is "unrideable".
Step back down to the jet that wouldn't rev out. Just focus on max rpm and wot. It will splutter elsewhere just hold it wide open.
There's not many chances to ride around lean without damage.

Yeah, that's.... I mean it's not exactly hunting for the idle but it's certainly taking its sweet time to find it.

Maybe your 122 isn't a proper 122. (Or are your main jets all from a reputable manufacturer from the same set?) Or maybe it's the idle jet that's causing it. Not sure which circuit has more influence on the coming down from high RPM.

Me, I've discovered a vast gulf between my 50/140 and 52/140 idle jets. Provenance unknown, both of them. The 52/140 is pretty darn rich, bringing on a honest-to-goodness plummet to idle. But with the 50/140, the idle comes more slowly, and at stoplights, the CHT climbs and climbs and climbs some more. Seems like way too much difference for an alleged 0.50mm hole vs. an alleged 0.52mm hole.

¯\_(ツ)_/¯

I'm kind of getting to the same place. Will probably try a 140 MJ too at this point.

I'm curious about your filter with open walls.
You removed filter material?

If I can repeat some blab:
The mj feeds all fuel to the engine. The ij is fed off the fuel well just above it, so a smaller mj will lean that circuit, as you can see.

The ac leans/richens the main stack, but it also influences when the circuit initiates. The 120 ac overlap with the ij circuit is really difficult to dial out, also risking one to use a too lean ij to compensate.

The SIP mj's I have are pretty accurate. You can mark a sewing needle with a marker and lightly twist it in, then go to your next +/- jet to confirm where they land in relation to each other. The 127 I'm riding with is a stamped SIP 130.

I have four 55/160 ij's. Each is different once installed. Unfortunately, they can't be reamed. They can be drilled, but I don't have micro-bits with that level of precision for the fuel end.
So KMT ij's, Lemarxon atomizers & slides, more money

Thanks for answer Ray. Really worth info.

Here about air filter.
I don't want going so deep with jets because some time ago it shows that all of them works and more or less proper. I mean if jet is stamped 130 it is obviously work richer than 128. Also in Ukraine not possible to get any jets fast - need order from SIP and wait for 2-4 weeks in the best case. So I prefer to work with what I have now.

My problem was that I read wrong the symptoms and go so low to 120 mj. Obviously too lean. Then I try 122 - bit better but still lean. Now I'm on 125 was rode whole day and tomorrow will too. Seems really good but we will look. At current time 125 seems is right for me or maybe 128.

I have no 140 jet. But with 135 it is absolutely pig rich. It is hard to start moving, hard to ride. It is impossible to start on the little uphill.
I realised that I need to test ride on every jet more longer as I did it before to understand what and how is change.

So now testing
AC 120
BE 3
125 mj
55/160 ij
Mix 2.5 out Everything seems to be ok here. I will continue to watch it.

Ok. Good. 135 unrideable. 125 could be close but I would be riding 130 for a test run. Short rides with bad jetting do little damage.

If there is no 3k splutter, you should add some. Until the main jet is decided. Fit a richer atomiser, like BE4, for now.

The BE4 is not a richer atomizer

And Roland87, if you promise not to tell Jack none of the usual suspects used for this level of tuning (BE3, BE5, BE2) are inherently rich or lean. They're fuel-flow brakes from initiation of the circuit to WOT.

The Lemarxon XX34 is somewhat the polar opposite of the BE4, and rich as F at half throttle on.

I had similar jetting woes where I could not get it to run well on any combo. This was all solved by ditching my LML filter and running either a pod filter on the front of the airbox, or just the bellows (no filter element). Maybe the LML carb-attached filter has particularly poor flow but I got a UNI pod for about the same price as what a Piaggio SI filter would cost.

If the air and roads are fairly clean in your location, I would try running just the bellows with no carb filter for a bit just to see.

Yes, LML blue filters is crap, too restricted. But I will stay with filter for sure. I had really madness when I trying to jet the SIP T5 air filter. Then I went back to original and all seems much better and clear.

Now I can't say that I have big problems with jetting thanks to all kind fellows here
On current jets scooter rides good enough. I just want be sure they everything works proper.

I realise you have just started an expensive love affair with lemarxon. I have been drilling my own atomisers for many years. But in this instance in Ukraine the BE4 will temporarily help.

Jack sorry for my stupidity but I can't understand your logic.

I'm riding now with BE 3 and 130 mj. It seems too rich. Engine won't revs higher than 5800-6000 rpm. It also have some random light jerks when opening from 3/4 to WOT.

The first one is where we thought your main jet was correct but as you went too lean at WOT, the lower rpm at 3k could possibly be way too lean. The second comment was after I heard your video.

To be safe while you're doing your main jet add some 3k richness to avoid cylinder damage. This can be changed after the correct main jet is known. Try 128 and see how that is.

So now when mj is too rich we can stay with BE 3?

Maybe. All the time it is too lean small damage is happening. Better to ride around richer low down until ready to adjust that part. BE4 for now.

Might need some richer pilot jets too. What you have?

IJ: 52/120, 50/120, 45/120, 52/140, 50/140, 55/160, 48/140, 40/120, 45/140, 42/140.

You are well stocked!

Thats why I hope it will possible to tune up right with what I have now.

Tried BE4 with 130 main. Obviously rich, engine response less. Bit harder start to move.
Then went to 128. Terrible. Feels leaner than BE3 125. Engine needs noticeable more time to go back to idle after blipping WOT. Absolutely not like it.

So maybe go back to BE3 125(or 128) and richer idle jet?

Turn out the mixture screwwww!

From Andre's (SIP) blog:
"For tuned engines, the position of the correct setting can be approx. between 1 and 5 turns."

But it works only on idle and a very early throttle opening?

Only worry about what happens at WOT and max rpm for the main jet. WOT is where the main jet is tested. Once you get the main jet number we can move on.

Are you sure your 125 128 and 130 are the relative correct sizes?

Going to need a richer pilot jet. For now as Ray said open the mix screw much more.

So no matter what is mixer tube for mj test on WOT?

I found that when I went from a BE4 to a BE3, it made the 138 jet I was trying out splutter at WOT. It wouldn't with a BE4. The BE3 richens the mixture at WOT while the BE4 leans it out.

BE 4 was installed to enrich the bottom. But now all feels much worse than before
So I keep trying.

It does change it slightly but less than a jet. Get the WOT test done quickly. The least time riding around with the wrong jetting the better.

Done it.
AC 120/BE4/130 is too rich. Engine splutters early and won't revs more than 6k.
Temperature also holds low.

AC 120/BE4/128 feels too lean. Rev up high but then taking much time to get back on idle. And have noticeable jerk when riding, for example, on 1/2 throttle then close it and then WOT.
Temperature increase fast and easy goes to 12º

I bit confused now. Maybe 128 jet is wrong? But as I remember it was all ok when I use it for jetting previous DR 177.

In my opinion the best combination was
AC 120
BE 3
125
55/160
Mix 2.5 out

Maybe need go back to BE 3 and try 130 and 128 mj ?

Going from 130 to 128 shouldn't make such a big difference as that.

Think your 128 is no good. As said, if you get a big sewing needle/ old carb needle/ something metal and tapered, the jets can be measuerd against each other. At least they will be in size order.

Do the same AC120/BE4 but with the 125MJ see if its better than the 128

I measured jets. Few times.
And 128 turns out bigger than 125 and smaller than 130. So all as it should be.

You mentioned that the 128 revved out…but that it took a long time to come back down to idle.

Wouldn't the idle jet size and mixture screw setting cause the revs to hang coming down and not the main jet size? So if 130 doesn't rev out at WOT but the 128 does…maybe that's the one?

I think about it too.
Now I put BE4/125 and will test it soon on my way from work to home. And then I try to play with mix screw and/or idle jet.

I have a tough time telling the difference between lean and rich sputter at low throttle. I found this video helpful:

I have also read in a number of places that low throttle rich burbling is normal on a tuned Vespa. The rich burbling has a deeper sound and when you open the throttle, the bike takes off. When it's lean, the sputtering has a different sound and when you open the throttle, the revs go up but there is not much power. Opening the throttle 1/3 to 1/2, the power kicks in suddenly and the bike takes off.

When I start scooter for going home from work is suddenly dies.
No spark. Changed plug and all turns ok.
So I can admit that WOT test of AC120/BE4/128 was wrong as spark plug was already bad.

So I done again.
AC120/BE4/125
Revs to 8200
https://youtube.com/shorts/Xp2WI90orL0?feature=share

AC120/BE4/128
Revs to 8000-8100
But it still terrible to ride on 1/4, 1/2 and 3/4.
https://youtube.com/shorts/nfcCUtm_ELQ?feature=share

Now its behaving normally.

When at full temperature the result will be a little different. For fully safe on the road the 128. For slightly quicker the 125. The 122 would be some peoples choice. I'd probably go 125 until there was a reason to change it.

Are you sure the mix screw is set correctly? And 2.5 turns was the result of adjustment. Not just set at 2.5. Sounds a bit off at low rpm.

After the pilot is better, you need to ride around at 4k rpm and decide if its rich or lean. A video of the rev counter while doing this will help.

No. I just put screw to 2.5 out without any adjustment. I read about it here and there and still can't understand how to do it. Maybe you have a good link?

How I can understand where I need to move with pilot jet?

Here's the simplest way I have thought of so far.
Warm the scooter engine up to over 60C (fine adjustment needs hottest riding temperature).
Turn the mixture screw 6 turns out.
Idle screw all the way in and then out one turn.
Start up. It should not idle. Be super rich and cut out without throttle.
If it doesn't cut out fit a bigger jet.
While running with small throttle, turn in the mixture screw until it idles. Set idle at 1500.
Leaving on fast idle. Set the mix screw until it runs the fastest. (if you miss the fastest point, it will get faster as it bottoms out. Go back and start again).
Switch off
If at 2.5 turns the idle jet might be correct.

Just to add from my own experience doing this;

It was hard for me to hear when the engine sped up turning in the screw until it was close to being fully turned in/bottomed out. You have to go slow as it takes time for the engine to register the change.

The idea is that if the screw setting ends up within something like 3/4 turns either in or out from the 2.5 turns starting setting, you have the correct idle jet.

The 55/160 is not a mild jet, and seems to have magical powers in terms of working (good enough) with 90+% of bikes at this level of tune, once the turns-out rule is tossed. The 52/140 also seems to have magical powers as a richer option but I have no clue why.

Running like shit once the main stack takes over is not an ij issue.
Ij issue is a bog/stutter from a rolling stop.
If you can't dial that out via the mixture screw in either direction you can use the best-I-can-get-it-to-accelerate turns-out as a guide for which direction to go with the ij.

A threaded CHT is a wonderful tool. My last jetting adventure was with a sensor under the plug. Twice the frustration.

"I'd" take it out with the current Jack stack, and a 140/BE3/125 and/or a 160/BE3/128 stack in your pocket, to see how it feels/CHT responds after a warmed-up trade-out.

You really need a dedicated amount of time to try your bits on the road, by the way. At some point, hours of trial and error and mixture screw adjustments from now, you will have temps under control and a rich but smooth (good enough) runner -- assuming no fuel flow or other issues.

By "I'd" I mean each tuned bike, rider weight, gearing, ride style, etc is unique. Only you can get to that point vs "this works for me" jetting.
Having said that, the main stack AC/atomizer combinations above are close to most I'm happiest setups at this level of tune.
Good luck!

Thanks a lot Ray!
Running like shit was caused by bad spark plug. It dies and I changed to new and all turns out ok.
And yes I'm trying to find time to test all on the road. Seems that I'm idiot and doing something wrong. Or there is air leak somewhere?
I do as you said Jack. But engine doesn't cut out with 55/160 or with 52/120. It just idle very high. Near 4500 with 55/160 and near 3800 with 52/120.

Then I try to do as Wiki says - Guide: SI Jetting Chart + How to Tune an SI Carb Ant it seems that 55/160 is lean.
Pity but I have no enough time for more tests and just put in 52/140.

Jetting now:
AC 120
BE 4
128 mj
52/140 ij
Mix 2.5 out
And looks like it runs ok.

I recorded few videos wit 55/160 and 52/140

AC120/BE4/128·55/160·Mix 2.5 out - 1st run
https://youtube.com/shorts/w3iVfNulyag?feature=share

AC120/BE4/128·55/160·Mix 2.5 out - 2nd run
AC120/BE4/128·52/140·Mix 2.5 out - 1st run
AC120/BE4/128·52/140·Mix 2.5 out - 2nd run