Pop, pop, pop, pop, pop sound normal for Polini box exhaust?
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Member
2005 Stella 2T
Joined: 14 Jun 2020
Posts: 11
Location: MA
Fri Jul 24, 2020 2:22 pm quote
Serious newb question.

The only performance mod Iíve made to my Stella 2T is a Polini box exhaust, plus a Piaggio predrilled airfilter, plus up-jetting. Iíve been very pleased with the increased power, especially the increased midrange power. First gear has become borderline useless, but otherwise the scooter accelerates strong and smooth to somewhere over 50 mph (Stella speedo provides only vague suggestions) where it will run happily all day long. That is 10 mph more than the speed limit on any road I am likely to be using.

The only question has been a consistent and prominent pop, pop, pop, pop, pop sound whenever the scooter is moving and the throttle is closed or nearly closed. Iíve messed around with jetting and timing and nothing has changed.

Is it safe to assume the pop, pop, pop, pop, pop sound is just the nature of the beast, and that no pistons are being harmed in the making of this movie?

Thanks.
Hooked
PX125E
Joined: 06 Oct 2018
Posts: 173
Location: Gravesend, Kent, UK
Fri Jul 24, 2020 2:31 pm quote


Last edited by worrywort on Mon Jul 27, 2020 3:16 pm; edited 1 time in total
Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
Joined: 26 Jan 2019
Posts: 1520
Location: california
Fri Jul 24, 2020 3:55 pm quote
Kowalski -
Some pop pop pop can be normal - but it can also be when you are a little on the rich side with idle jet.
Likely will not hurt you - but may leave you a little less than crisp at 1/8tth-1/3 throttle.

Are you happy with its pop down there or is there any bog?
Did you upjet only the main - or change other? What is your jet stack?
Are you getting any 4 stroking (that's like a machine gunn sound) when you hold 1/8th throttle and are cruising?

If it is running smooth on the first bit of the throttle and not bogging - you are fine. But if you are getting 4 stroking and its not very crisp there - and you are getting pop pop pop when you let off the throttle and coast down - you are likely a bit rich on the idle.

Answers to above and a pic of the plug where the outer ring at the bottom of the threads is showing - would give more insights.

My $.02
-CM
Member
2005 Stella 2T
Joined: 14 Jun 2020
Posts: 11
Location: MA
Fri Jul 24, 2020 4:46 pm quote
Interesting. My research suggested the pop, pop, etc. was caused by a lean condition. Partly for that reason, and partly because I wanted to be conservative while I dial things in, I have been running what I think is a little rich on the mixture screw (2 and 1/4 turns out when just 2 probably would be optimal). Still plenty crisp and no bogging.

Jetting is: idle jet--55/160; air tube--160; mixing tube--BE3. I have tried main jets from 105 to 108 without noticing much difference; perhaps a little bogging with the 108.

No 4-stroking as far as I can tell.

I think the plug is too new for a photo to be instructive but, if anything, the outer ring is reading lean. Is it plausible that I would need to go higher than 108 on the main jet after changing only the exhaust and the air filter?

I forgot to mention that I went a little cooler on the spark plug too. For some reason, Genuine specified the equivalent of an NGK B6ES. I think that is too hot, at least for summer use, and am running an NGK BR7ES now (B7ES is discontinued).
Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
Joined: 26 Jan 2019
Posts: 1520
Location: california
Fri Jul 24, 2020 7:37 pm quote
Kowalski,
- Generally - best to dial in the main before the idle. they have an effect on each other - and you can end up chasing your tail.
- The best method is to go up in main jet size until the bike won't rev out when you pin it WOT in 3rd. It will climb to about 5500 or 6000 RPM but just won't want to wind up to 7500-8000+. That is your starting point for walking back down the jet size.
- exhaust is probably between 5 and 10 points of increase main jet size - though I would guess it will end up closer to 5 then 10.
- Any change to the carb air supply can have a big effect - closer to 10 than 5.
- Changes can be cumulative.

In a nutshell:
- Focus on the main first. Go big enough for it not to rev out as noted above - then start walking down. You will feel a jet that makes it take off. That is the one - maybe one smaller.
- Lean idle jet will show itself by returning to idle slowly, or hunting - seeming to come to idle - then dropping again.
- Rich idle jet can be less crisp - and can go pop pop pop when you decelerate from a rev. The rim of the plug will also get very black.

One quick test to see if you are close on idle jet: set idle to about 1200-1500 RPM - and then screw your idle jet in - when you reach all the way in (don't over tighten) it should stall. if it just keeps running happily - you are probably too rich.

If you want more help - let us know what jetting you had before you made the change to 105/108 main - to give us a bit more info.
Good luck with it!
-CM
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2226
Location: London UK
Fri Jul 24, 2020 10:28 pm quote
Kowalski wrote:
Jetting is: idle jet--55/160; air tube--160; mixing tube--BE3. I have tried main jets from 105 to 108 without noticing much difference; perhaps a little bogging with the 108.
This jetting is not original for your scooter. And will not run properly. Ever. Someone put a P200 carb on it?
Member
2005 Stella 2T
Joined: 14 Jun 2020
Posts: 11
Location: MA
Sat Jul 25, 2020 5:14 am quote
All changes were made by me.

Carb is the stock Spaco 20/20.

Stock jets were: idle jet--40/130; air tube--140; mixing tube--E3; main jet--94. Remember, these came with catalytic converters and were tuned to meet US emissions laws.

The new idle jet--55/160, air tube--160, and mixing tube--BE3, is the so-called Bald John jetting which, as I understand it, is just the stock jetting for a European Vespa PX 150. Many people have reported good results using that plus a main jet around 103 with the SIP Road exhaust and a drilled air filter. I started with a 105 main jet just to be safe.

The scooter has run well since the changes. I just didn't know what to make of the pop, pop, etc. sound it made when the scooter is moving and the throttle is closed or nearly closed. Is it just the quasi-expansion pipe box exhaust version of the ring, ding, ding, ding, ding you get with a true expansion pipe, or something more sinister? Fiddling with the mixture screw, the timing, the spark plug and the main jet has had zero effect on that.
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2226
Location: London UK
Sat Jul 25, 2020 5:58 am quote
Kowalski wrote:
All changes were made by me.

Carb is the stock Spaco 20/20.

Stock jets were: idle jet--40/130; air tube--140; mixing tube--E3; main jet--94. Remember, these came with catalytic converters and were tuned to meet US emissions laws.

The new idle jet--55/160, air tube--160, and mixing tube--BE3, is the so-called Bald John jetting which, as I understand it, is just the stock jetting for a European Vespa PX 150. Many people have reported good results using that plus a main jet around 103 with the SIP Road exhaust and a drilled air filter. I started with a 105 main jet just to be safe.
What you are feeling is that this jetting is not appropriate for yours. Lucky you haven't blown it up.

Put all the original jetting back but up the main jet with your 103. This will work.
Member
2005 Stella 2T
Joined: 14 Jun 2020
Posts: 11
Location: MA
Sat Jul 25, 2020 6:58 am quote
Update:

Based on CM's input, I reinstalled the original 40/130 idle jet long enough to confirm that it eliminates the pop, pop, etc. sound.

In the meantime, Jack has weighed in that I should just stick with that. But, I worry that will be too lean for the Polini Box plus drilled air filter. Should I be looking for an idle jet somewhere between the 40/130 and the 55/160? I'll do my own search, but is there a handy guide for dialing in the perfect idle jet?

Thanks for you help so far.
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2226
Location: London UK
Sat Jul 25, 2020 7:59 am quote
Get the main stack changed back to the original as soon as. It will seize on the first long journey if you don't.
Member
2005 Stella 2T
Joined: 14 Jun 2020
Posts: 11
Location: MA
Sat Jul 25, 2020 8:33 am quote
Jack221 wrote:
Get the main stack changed back to the original as soon as. It will seize on the first long journey if you don't.
What about the mixer tube? Go back to E3 or can I keep the BE3? Asking mostly because I see the BE3 is part of the "Jack Stack."

Thanks.
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2226
Location: London UK
Sat Jul 25, 2020 10:58 am quote
For yours the E3 will be ok with the 140, a BE5 would also work. If you want to buy an AC120 then use the BE3.

If you have a camera take a picture of your piston crown through the plug hole. Check to see if you've done any damage.
Member
2005 Stella 2T
Joined: 14 Jun 2020
Posts: 11
Location: MA
Sun Jul 26, 2020 4:16 am quote
I'm not equipped to take a good photo of the piston crown. What I can see looks ok.

This dialogue has been very informative. I rode around some yesterday with the 40/130 idle jet and 140/BE3/105 for a main stack. I can now see the 55/160 idle jet I was using was too rich and the 160 air tube I was using was making the main stack too lean. With the 140 air tube and 105 main jet, the main stack is now too rich at WOT. I have to order some smaller main jets before I can address that, but the scooter still is pretty rideable in the meantime.

I haven't seen anything that explains the numbering for the mixer tubes or how they relate to the air tubes. I will try going back to the E3 mixer tube today and see if I can detect any changes.

Thanks for the help guys.
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2226
Location: London UK
Sun Jul 26, 2020 4:58 am quote
Instead of buying jets you could grind a bit out of your exhaust port. Increase the power to match the 105
Member
2005 Stella 2T
Joined: 14 Jun 2020
Posts: 11
Location: MA
Sun Jul 26, 2020 5:51 am quote
Jack221 wrote:
Instead of buying jets you could grind a bit out of your exhaust port. Increase the power to match the 105
Hmmm. I may not have a borescope, but I do have a Dremel.

But seriously, the Polini box and the drilled air filter should be adequate for my intended use, which is just putting around my little seaside village.
Molto Verboso
1963 VBB2T
Joined: 07 Nov 2012
Posts: 1919

Sun Jul 26, 2020 8:03 am quote
In the big bike world I have learned that free'er flowing exhaust will add to the popping and most times a fuel tuner will minimize the popping. Also a leaking exhaust at the joint will cause for popping.
Member
2005 Stella 2T
Joined: 14 Jun 2020
Posts: 11
Location: MA
Mon Jul 27, 2020 11:44 am quote
'Nother update.

Putting the E3 mixing tube back in was an improvement. As far as I can tell, the 140/BE3 combo made the scooter too lean at mid-throttle and the E3 richened things up, which made for smoother mid-throttle performance.

I think I understand how to dial in the main jet, so I feel like I'm almost there now. My only remaining doubts are whether I am leaving anything at all on the table by using the stock 140/E3 air/mixer tube combo and 40/130 idle jet?

Apparently, a BE5 mixer tube is more commonly used with a 140 air tube. Given that the object of this exercise is to optimize midrange power with no mods other than the Polini box and the drilled air filter, and that I am not concerned about the cost of a few jets, would the BE5 mixer tube be an upgrade, even if just a little?

Likewise, I see there is a 42/140 idle jet, which has a similar ratio to the 40/130 (3.33 vs. 3.25) but, if I am understanding things correctly, can flow a little more of a charge and might be more compatable with a 140 air tube. Would a 42/140 idle jet be an upgrade, even if just a little?

Also open to suggestions around the 120/BE3 "Jack Stack," or maybe a 130/BE(fill in the blank).

Thanks for your patience.
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2226
Location: London UK
Mon Jul 27, 2020 1:24 pm quote
Get the main jet correct and see how it rides. Then decide. Without tuning, stock jetting with a slightly bigger main is all that's needed.
However, if you do want to go faster, a bit if dremelling here and there is all it takes.
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