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**SOLVED**

Hello, I have been reading topics about basic of jetting, but still confused to choose the right Main jet and Idle jet stack because still lack of experience in Vespa Scooter, especially for my Super 150 (VBC1T). I have the carb specs below from previous owner:

- Dell'Orto SI Carburetor 20/15D
- Idle jet (size 140, the owner put something to close/clog the upper hole)
- Main jet stack (I can't figure out the size of air corrector and atomizer because there is no number on it, maybe it's eroded by lots of scratches. The main jet has a number, but I can't see it clearly so i think its 90 or 92)

Is it correct if I use stock Main and Idle Jet stack? Each size is:

- Main jet stack (120, E-1, 88)
- Idle jet (42 without upper hole)

I also have 3 main jets (90, 92 and 95). Please I need good recommendation thank you
⚠️ Last edited by xperimental21 on UTC; edited 1 time
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Here are the stock specs for a VBC1T:
https://www.scooterhelp.com/scooters/VBC.150super.html

Is the engine + pipe stock as well? That's key information, as usually the jets would already be stock on a stock scooter
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xantufrog wrote:
Here are the stock specs for a VBC1T:
https://www.scooterhelp.com/scooters/VBC.150super.html

Is the engine + pipe stock as well? That's key information, as usually the jets would already be stock on a stock scooter
How to know if the engine is stock? and which engine part?
I think the pipe is stock as well, but there's something inside is removed so the pipe sound really loud
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You should post pics here of what you're working with - but I guess the first thing I'd look for is the "piaggio" stamping (a little leaf-shaped symbol in a hexagon) on the cylinder and exhaust. If it's there it is likely stock. Could check the engine number as well in case it's "all piaggio" but appears to be an engine swap (i.e., not a VBC motor)

*edit* also, if you add where you are located to your profile, there might be someone local who can help you too
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xantufrog wrote:
You should post pics here of what you're working with - but I guess the first thing I'd look for is the "piaggio" stamping (a little leaf-shaped symbol in a hexagon) on the cylinder and exhaust. If it's there it is likely stock. Could check the engine number as well in case it's "all piaggio" but appears to be an engine swap (i.e., not a VBC motor)

*edit* also, if you add where you are located to your profile, there might be someone local who can help you too
I am new to scooter world, and never own a bike before. Rather than messing with the engine to know if both still stock, so I checked the frame and engine number.
The frame is VBC1T 17052
The engine is VBC1M 14414

So the engine had been replaced right?
Frame number
Frame number
Engine number
Engine number
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Actually the engine and frame numbers won't match, so it's good/fine that both are VBCs. I brought that up because there's always a chance someone dropped a P-series motor in or something.

Both being stamped VBC still doesn't tell us whether anything has been done to the exhaust or cylinder to make it not-stock, however.

Is there a reason you are working on the jetting? I'm trying to figure out if something was wrong/there are any clues that you should not assume it is stock.

I do suggest you take the time to examine the exhaust pipe and cylinder setup, because whether you are having problems right now requiring jetting or not you 1) will definitely need to be comfortable working on them at some point anyway and 2) really can't get too far with a vintage scooter if you don't know what engine setup you have. Taking the cylinder head off is easy (just a few nuts; just be careful not to overtighten them when you put it back on), and you can probably spot the piaggio stamp on the exhaust without taking it off (although I don't have a VBC myself, so I'm not 100% where it would be on it).

More pics! 99% of performance exhaust pipes will be immediately recognizable as not-stock, for example.

Cheers
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xantufrog wrote:
Actually the engine and frame numbers won't match, so it's good/fine that both are VBCs. I brought that up because there's always a chance someone dropped a P-series motor in or something.

Both being stamped VBC still doesn't tell us whether anything has been done to the exhaust or cylinder to make it not-stock, however.

Is there a reason you are working on the jetting? I'm trying to figure out if something was wrong/there are any clues that you should not assume it is stock.

I do suggest you take the time to examine the exhaust pipe and cylinder setup, because whether you are having problems right now requiring jetting or not you 1) will definitely need to be comfortable working on them at some point anyway and 2) really can't get too far with a vintage scooter if you don't know what engine setup you have. Taking the cylinder head off is easy (just a few nuts; just be careful not to overtighten them when you put it back on), and you can probably spot the piaggio stamp on the exhaust without taking it off (although I don't have a VBC myself, so I'm not 100% where it would be on it).

More pics! 99% of performance exhaust pipes will be immediately recognizable as not-stock, for example.

Cheers
I really appreciate you help, I am going to check the pipe and cylinder tomorrow and post the result.

Anyway, this is the main reason why I want to "correct" the jetting (pic below), even the mixture screw wasn't turned out.

beside that, I often experience which the carb is popping or blowback on high revs, I don't know what is the correct word to describe the problem. Someone said that could be from the timing, but the previous owner had set it correctly.
Fouled spark plug? NGK B6HS
Fouled spark plug? NGK B6HS
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xperimental21 wrote:
beside that, I often experience which the carb is popping or blowback on high revs, I don't know what is the correct word to describe the problem. Someone said that could be from the timing, but the previous owner had set it correctly.
I think the sound you are describing is rich bogging on full throttle, which might be a clue as to why the plug is black. Im an amateur with engines, but i would be heading to that carb, from experience of DIY issues, id be doing the following. IMO you want to clean the carb out and plenty of knowledge on here to help you do that. Or head to youtube and watch step by step instructions of a carb teardown. When you take it off, ensure you block the inlet hole up with a rag so nothing drops in there and the peeps on here can show you the way regarding jets etc. I bet its full to brimming with debris. Is your air filter in order too, clean? If its clogged it will lead to lack of air = rich.

Might well all be in order, just needs some routine TLC, cleaning and replacment of cheap components, such as jets if the guys on here indicate they are wrong. If the air hose to the carb box mounted correctly? Is the air hose split? It could well be something else, but hopefully if its just the carb its gonna be a very cheap fix!
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ferriswolf wrote:
I think the sound you are describing is rich bogging on full throttle, which might be a clue as to why the plug is black. Im an amateur with engines, but i would be heading to that carb, from experience of DIY issues, id be doing the following. IMO you want to clean the carb out and plenty of knowledge on here to help you do that. Or head to youtube and watch step by step instructions of a carb teardown. When you take it off, ensure you block the inlet hole up with a rag so nothing drops in there and the peeps on here can show you the way regarding jets etc. I bet its full to brimming with debris. Is your air filter in order too, clean? If its clogged it will lead to lack of air = rich.

Might well all be in order, just needs some routine TLC, cleaning and replacment of cheap components, such as jets if the guys on here indicate they are wrong. If the air hose to the carb box mounted correctly? Is the air hose split? It could well be something else, but hopefully if its just the carb its gonna be a very cheap fix!
Oh bogging, i see... I had cleaned the carb and changed some components last week, even the air filter/cleaner is brand new. I am going check everything in case if there is something I missed.
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Hopefully now somebody will be able to comment on your jet stack and see if that is the issue. I hope it’s just a case of weakening the jet to lean the mix it down a bit . Once a plug is fouled like that you really need a couple of new ones as you will have to experiment with jet changes, take out for different runs of speed, eg just pottering around town, wider throttle runs then examine your plug to see if colour changes to more of a chocolate brown. It could be it was jetted higher in the main at some point for a different exhaust and now that exhaust has changed . Hoping it’s a jet issue as it’s a cheap fix but takes time getting it right. Let’s hope the experts on here can get you there. Took me a couple of weeks experimenting on my px as had similar problems, black plug at low mid speeds but lean plug on high speeds and that gurgling sound on carb when it was too rich . It’s like balancing a see-saw but it will be ok I’m sure
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I have checked the cylinder head, but It wouldn't came out, there was a paste-like gasket inside the gap. But i found P/ logo on the head
External inline image provided by member with no explanatory text

The exhaust is so rusty, no Piaggio or another brand logo on it. I saw a kind of serial number which was barely visible on the exhaust side.
External inline image provided by member with no explanatory text
External inline image provided by member with no explanatory text
External inline image provided by member with no explanatory text

After cleaned the carb, replaced the spark plug, used stock main jet stack, then the mixture screw showed a response to RPM so I turned the screw out while hearing the rpm and found the best setting. I tried to drive it around, but as soon as the throttle was fully opened, the carb was bogging (sounded like small explosion from the venturi). I tried to set the mixture leaner and richer, and the problem still there even swapped the main jet to the older one.

Could be a problem from the Ignition? I give up. I am going to replace the contact breaker with CDI unit, hope the problem will gone.
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ferriswolf wrote:
Hopefully now somebody will be able to comment on your jet stack and see if that is the issue. I hope it’s just a case of weakening the jet to lean the mix it down a bit . Once a plug is fouled like that you really need a couple of new ones as you will have to experiment with jet changes, take out for different runs of speed, eg just pottering around town, wider throttle runs then examine your plug to see if colour changes to more of a chocolate brown. It could be it was jetted higher in the main at some point for a different exhaust and now that exhaust has changed . Hoping it’s a jet issue as it’s a cheap fix but takes time getting it right. Let’s hope the experts on here can get you there. Took me a couple of weeks experimenting on my px as had similar problems, black plug at low mid speeds but lean plug on high speeds and that gurgling sound on carb when it was too rich . It’s like balancing a see-saw but it will be ok I’m sure
I did everything in order for problem to be fixed. Maybe the problem is from the iginition timing or the old contact breaker. I am going to replace it with CDI unit at a local Vespa workshop.
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I think your scooter is stock.

I think the timing needs to be done over - wouldn't assume the previous owner had it right. Stick with stock jetting on a stock scoot, and see if you can get the timing dialed in - that plug is probably fouling due to ignition and not the scoot somehow becoming way too rich
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xantufrog wrote:
I think your scooter is stock.

I think the timing needs to be done over - wouldn't assume the previous owner had it right. Stick with stock jetting on a stock scoot, and see if you can get the timing dialed in - that plug is probably fouling due to ignition and not the scoot somehow becoming way too rich
Yep I am going to get It done, anyway could a fouled plug be reused?
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xperimental21 wrote:
Yep I am going to get It done, anyway could a fouled plug be reused?
I would say no, bin it. You should buy a few IMO for experimenting. They are as cheap as chips on ebay.
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So I solved the problems. Anyway, thank you guys for the responds.

1. The fouled spark plug [solved]
Solution: I cleaned the carb, and changed the mixture screw, and turned it 1.75 out richer. You can see the result from my latest post in this forum about spark plug condition.

2. Worst bog issue with small explosion on WOT/High RPM [solved]
Solution: The contact breaker's gap was too wide, so I adjusted it to approx. 0.5 mm which is the best gap for my engine.
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Glad you got it running better. Keep in mind that changing the breaker gap alters the timing. I would just verify that if I were you.
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Moto64 wrote:
Glad you got it running better. Keep in mind that changing the breaker gap alters the timing. I would just verify that if I were you.
Oh I see, actually I was going to check and fix the timing but I didn't have proper equipment and the process is complicated for me as a newbie. Then I fix the breaker gap as temporary solution.
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Is there not an electronic ignition kit available for that model or is it a real faff uprating other parts due to 12v being introduced ?
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