Modern Vespa

Thanks a lot to all for the help.

I put in:
AC 120
BE3
MJ 122
IJ 55/160
Mix 2.5 out

Runs better than previous set up. But now appeared distinct jerks at low speeds. When I ride with hold 1/4 throttle or less on low speeds and then open throttle the engine reacts with a noticeable delay and then accelerate nicely. Sometimes the same was when I changed gears to highest(Sorry for my bad english I hope you understand what I'm trying to say)

Changed main jet to 125 and tomorrow will try again.

I'm not sure but it seems that I'm on the right way.

I ride near 150 km with
AC 120
BE 3
MJ 125
IJ 55/160(2.9)
Mix 2.5 out

Plug looks better but still have some jerking when open throttle from 1/4. Seems to me as idle jet is too lean. Today changed it to 50/140(2.8), and try how it goes tomorrow.


AC 120, BE 3, mj 125, ij 55/160(2.9), mix 2.5 out

AC 120, BE 3, mj 125, ij 55/160(2.9), mix 2.5 out

AC 120, BE 3, mj 125, ij 55/160(2.9), mix 2.5 out

I think you will end up with 45/120, having tried many pilots on my DR and mix adjusting this is where it's finally happy. The jerking sounds like not enough fuel imo and what I experienced as pilot too lean.

Which main jet size made it splutter at WOT?

Sorry but what is "spluttering"? It is effect when main jet is too rich and engine can't revs out?
If so then I don't know. I never try so big main jet. But as you can see from my first message in this topic all was ok with:
AC 160
BE 3
MJ 125
IJ 55/160
With spark plug 9.
When the outboard temperature dropped to 5-10ºC / 41-50ºF I just changed main jet to 128 and spark plug to 8.
Never had problems in whatever hard modes I used the scooter no seizes or problems with temperature and spark plug always have nice right color.

Today I ride 15km with 50/140(2.8) idle jet. Obviously it better than 55/160(2.9)
But there are still jerks as before although they are weaker.
I hold 88-95 km/h for the 4-5 km on the highway with slight uphill and engine temperature not gets above 122ºC / 251ºF. Outboard temperature was near 8ºC / 46ºF.

If you don't know the main jet splutter jet size, then it will be difficult to guess the right jet.
Yes, increase the jet size until it won't rev out. If you can't make it splutter then something is really wrong.

I think the jerking is coming from the mixer tube being too lean. Did you get a BE5 yet? Try that.

I also think the AC120/MJ125 combination is too rich for your set up and your sparkplug photos agree. I would expect you to get lots of splutter at wide open throttle with that jetting. You either need to go bigger on the air corrector or smaller on the main jet, but only change one at a time so you can track cause and effect.

I received BE5 today, few hours ago. Few days later I come back to my P.
Thanks for advices guys.

I am following your thread with interest I may have the same condition you describe so well with the delay and everything else seems to be just fine. Wondering if it could be a lean slide condition. Wish there was a 3D printable cap that could be pushed into the vertical slide duct.

Oopsclunkthud might be able to do this and make it available through Shapeways, like he has for the venturis. I'd order a few for sure!

I managed to achieve the same thing with a rubber bung cut out of the soft rubber in an old sparkplug cap. Airtight and doesn't break and takes 3 minutes!

Please let me add to my suggestion that go back to your Prehistory, which was pretty textbook.
Warning, I have no personal experience with this:

The T5 filter on your carb raises the height of the intake. The bore of the carb(24g) that it was designed for is different. It will be VERY difficult to jet for smooth acceleration at first throttle.

That go-faster filter/cover is made for 20 or so+ hp bikes, not daily ridden/touring bikes with your level of tune. Its effect is not linear through the rev range, making jetting it all more complicated, as you can see

Don't try to jet around a fixed 2.5 turns out on the mixture screw. The impact of properly adjusting that screw is big. And don't focus on the pilot circuit until you find your correct mj.

By the way, I've never seen a crush washer as crushed as yours!
It's no longer doing its job as a seal there.
"If I were you" I'd base jetting more on a combination of feel, sound, and CHT response vs plug peeks that show a combination of your ride.

Great advice. Your explanation made the problem clearer to me.
The weather is getting colder every day and I only ride my scooter when I need to.
Perhaps I will practice a little more with all these jets, and after the winter I will return to the old settings. Maybe I'll just leave the Polini lid.

And what is crush washer? Washer on spark plug? I don't use it because I have CHT ring on the plug. Or you recommend use washer any way?

At least you have a setup you were happy with to fall back on, in case your brass bit experiments don't work out!
I couldn't jet the Polini venturi kit (the one that comes with that stupid filter) for a daily rider bike. On a 24 carb it's not fun try to sort out on a moderately-tuned setup -- maybe on a 20-20 carb or a rally bike things would be different, but I don't have any experience there.

Thought it was a crushed crush washer in your photo, with that oil down the threads.
A crush washer will distort the soft copper sensor after a while, preventing a seal. I have spare sensors so I just leave the washer. It's easier, seals with less torque, and has no affect on CHT readings.
Pretty sure that's why they recommend removing it, though. Supposedly using both also lowers the electrode in the bowl, but I can't ride-feel any difference.

Good luck with the brass bits!
As Jack221 mentioned, sort out the main jet before the pilot circuit. Otherwise you'll be jetting in circles.

Hi again!
So I followed to ferriswolf's advice and put in this jets:

AC 140
BE5
128
45/120(2.66)

Seems it best set up from all what I tried before. Engine temperature is nice, scoot accelerates really good and follows the throttle very responsively, riding is smooth, no any jerks.
Here is plug colour after 200 km. As for me it is bit rich, so I can go down in jets.
What do you think friends?


AC 140, BE5, 128, 45/120(2.66)

AC 140, BE5, 128, 45/120(2.66)

AC 140, BE5, 128, 45/120(2.66)

AC 140, BE5, 128, 45/120(2.66)

Your main is bit too high , I'd drop to 125 where mine is now however if you are facing cold weather soon I'd leave where it is as it's gonna lean up in 3-4c temps. I'd say your plug looks fine to me, better bit rich than rudely spat off with seize.

Put in 125 main. And give it a good highway run for 115 km. But engine temperature rising very fast. If I hold 85-90 km/h for a 1-2 km temperature gets to 150ºC and continue to rise.
Outboard temperature is 13-15ºC.
So I think that 125 mj is too lean. This evening put back 128 mj and a bit learner idle jet.
Now it is:
AC 140
BE 5
128 mj
52/140(2.69)
Mix 2.5 out
Tomorrow I will test it.

And here is plug with AC 140, BE5, 125, 45/120(2.66)


AC 140, BE5, 125, 45/120(2.66)

AC 140, BE5, 125, 45/120(2.66)

AC 140, BE5, 125, 45/120(2.66)

Why have you moved away from the 120 air corrector? Objective is to jet with the minimal air correction.

I can't get proper engine work with AC120. You can read about it in my earlier posts. Ferriswolf advised me another stack and it was near perfect as for me.

Lean jetting runs lovely. Smooth, nice, responsive. Downside is it wears out the engine ten times faster. If you like rebuilding engines and helping the German economy, carry on.

But I don't aspire to ride on lean jets. I trying to find right combination.
As you can see I rode on AC 160 - BE3 -125 mj - 55/160 idle - mix 2 out near 12000 km without any issues. And I never spared the engine, squeezed all the juice out of it for long hauls in the heat at 85-90 kmh, mountain serpentines all day long and at full throttle.
When weather becomes cold I change the main to 128 and everything is OK.

Or you mean that I must stay with AC 120 and find right jets combination with it?

Oh I just read your signature again and realised that I don't tried that stack. Is it worth to try AC120 - BE5 - *some main jet* in my case?

I know your question was directed at Jack, but if I might chime in with some observations after having followed his BE5 + 120AC advice -- it works.

The problem as I understand it is that a big(er) air corrector and a mixer with bigger holes up top is that you end up with a too-lean situation at open throttle. All is fine for ripping between stoplights around town, but once you're out on the open road and really ragging on it all day, all kinds of unpleasantness starts to happen.

Allowing less air into the mixer tube might sound counterintuitive, but on the SI carb the end result with a moderately tuned setup like yours (and mine) is that you achieve a more consistent fuel-air mix at all throttle positions. And no pinking, runaway CHT, etc when you're going full tilt for long periods.

For the record, with the 120AC & BE5 on my 221cc malossi sport, with T5 filter, the sweet spot seems to be a 128 main jet. In this cold weather that is. 125 when it's hot out.

No problem Jim for sure. I am grateful to everyone and appreciate all the advices.
But AC 140 - BE 5 - 128 - 45/120 is seems very close to perfect IMHO. Maybe bit rich. CHT and all other aspects was OK.

I like this schematic

Ohhh, me too!

Great.
Nothing wrong with a bit rich!

The BE5/140/mj is a popular combination lately.

How does this setup compare to Prehistory on the road?

It your scoot is behaving how you want , performance , temperature , correct plug colour across all running conditions why try change that ? If it ain't broke leave be. Really you have found the answer to your experimentations , tried a plethora of jet combos with helpful advice from all and some worked out, some didn't and you get to where you are now satisfied . I think if you had a jet guru standing over your bike jetting it with ac120 or whatever and spending day riding out plug chopping you might well get this optimum setup but from what your reporting it seems you have found it. The DR imo is nothing more than a big bore cc upgrade which gives a bit more pep for commuting, that jet setup you have works for the performance it offers. . I reached similar top speed when mine was a 125 with a road 2 pipe and drilled filter, this doesn't need to be complicated now if you state your scoot is not overheating, running well and plug indicating it's fairly happy. In my book that's job done .

I like it more than Prehistory.
Yes with Polini box I loose 5-7 km/h of top speed but I agree to sacrifice it for those smoothness and strong acceleration. Yes Prehistory was run perfect as for and that fact that it runs more than 12000 km is talking about it too. But scoot was really much more nosier and more vibrations. Bit more top speed and noticeably less acceleration.
The one thing what bit confuses me is plug colour - now it is noticeably darkest than on Prehistory. Thanks for your great advices! Also want to thanks all of you guys who helps me in this thread to find right combination. You all are awesome!

Yesterday, as I mentioned above, a go back to 128 main jet and I also put in 52/140(2.69) idle jet. Today I give it a good ride and it feels better than with 45/120(2.66) idle jet.
So now jetting is:
AC 140
BE 5
128 main
52/140(2.69) idle
mix 2.5 out

P.S. This is not end of thread and I will continue to watch and share the results of how its going.
Big thanks one more time! I'm really happy


Just a photo from today ride out.

Seems like all is sorted. If it feels and sounds good it usually is. The basic colours on a 2 stroke plug that do damage are grey, silver and white. All shades of brown and dry black are acceptable. Signs of being lean are, rough running, vibration, slow acceleration, the feeling it's revving really high when it actually isn't so much.....and many more. Have some fun riding it and see how to it goes.

Hi friends!
Here is some updates.

This is how plug looks after near 300km. Outboard temperature is now 5...10ºC.
AC 140
BE 5
128 main
52/140(2.69) idle
Mix 2.5 out

As for me it still bit on the rich side.
But I have nothing to do as 125 main jet is too lean(tried before).
Now I screw in mixture screw and now it is 2 turns out.
Maybe put leaner idle jet?

Nevertheless scoot runs great without any problems.


AC 140, BE 5, 128 main, 52/140(2.69) idle, mix 2.5 out

AC 140, BE 5, 128 main, 52/140(2.69) idle, mix 2.5 out

AC 140, BE 5, 128 main, 52/140(2.69) idle, mix 2.5 out

Do you have drilled air filter? You could try standard filter on 125 jet , the drilled filter needs about 2 pts over standard , I think this will balance you out for browner plug . So you are placing yourself in between 125 and 128 by reducing air over jets on standard filter , just an idea that might be spot on .

I have SIP T5 air filter now.
With hole above the idle jet. No hole above AC and there is hard to be drilled as there is mount for filter top.

Did you test BE2 with the 120AC? Color doesn't look brown. Maybe need 130AC?

I don't have BE2.
I think that I stay with what I have now, maybe play with idle jets and mixture screw. Weather is really cold now and gets colder with every few days.
If I order now BE2 it will arrive in 3 weeks - I just won't be able to test everything again. Because it will be too cold.
It is time to park my P for winter and get out my four stroke LML which already have fresh winter tyres and other stuff for winter riding.

If it's running well don't try and fix things that don't really need fixing. That plug could change colour easily rich just on the pilot and low speed running . If you can WOT it safely that's what it's all about imo, what is it like when you floor it at 55mph plus then ignition off plug chop?

Plug is too grey looking.
AC120/BE3 to check what size the main jet ends up. Big main jet to make heavy WOT splutter, then reduce one size at a time.

Really?
Maybe it is something with color rendering on different laptops or smartphones?
The plug colour is very close to this one - AC 140, BE5, 128 mj, 45/120(2.66) ij, mix 2.5 out

Also have some update. The last one in this year as now already really cold and icy weather. Hope in next year I can get you again with my stupid questions, but about VMC Super G jetting

I turn mixture screw in for 0.5 turn, now it is 2 turns out.

So the jetting is:
AC 140
BE 5
128 main
52/140(2.69) idle
mix 2 turns out

And here is plug colour


AC 140, BE 5, 128 mj, 52/140(2.69) ij, mix 2 turns out

AC 140, BE 5, 128 mj, 52/140(2.69) ij, mix 2 turns out

AC 140, BE 5, 128 mj, 52/140(2.69) ij, mix 2 turns out