OP
@xperimental21 avatar
UTC

Hooked
Super
Joined: UTC
Posts: 100
Location: -7 degrees
 
Hooked
@xperimental21 avatar
Super
Joined: UTC
Posts: 100
Location: -7 degrees
UTC quote
When the throttle closed the revs come down but also with slow "popping/very slow idle" sound then back to normal idle after few seconds (the idle when the scooter stands still). This also happens when I go downhill.

I saw most vespa scooters here (most are P series) have nice smooth decreasing revs sound when the throttle was closed even after fast acceleration.

Is it normal for older vespas to have that kind of down revs sound?

My scooter:
Vespa VBC1T Super
- Modified stock IGM exhaust so it "breathes" freely
- Stock 20:15D dellorto carb
- Idle jet 42-140 with 2,5 screw out, before was 42 no air corrector with 1,8 screw out
- Main jet 160-BE3-102, Up jetted from 98 main because the WOT was lean (the electrode was cream white)

Someone said it was from the idle circuit, which is too lean. But It still happens, even tough I use richer (42) idle jet with mixture screw set richer, also swap to leaner idle jet to get more screw out from the mixture screw.

This is how it sounds, sorry if the video is only when the scooter on stand. But this is what it sounds like when the throttle closed
https://www.youtube.com/shorts/3T4T9XhyZ4I

Thank you
@chandlerman avatar
UTC

Lucky
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 79 P200E, 66 Lammy S3
Joined: UTC
Posts: 9838
Location: Nashville

47 Days Since Last Explosion
 
Lucky
@chandlerman avatar
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 79 P200E, 66 Lammy S3
Joined: UTC
Posts: 9838
Location: Nashville

47 Days Since Last Explosion
UTC quote
Usually, that popping and hunting is indicative of a too-lean idle jet.

Is this a new thing, or has it always been like that. If it's new, start by cleaning the idle jet. If that doesn't fix it, you may have picked up an air leak somewhere. If it might be an air leak, don't ride it hard until you can get it sorted, as that's a great way to seize a bike, especially coming off the throttle if you're already lean.

If it's not, then you need to get a richer idle jet. Try a 52-120, maybe grab one either side of that, and it should sort you out. You can also try taking the mixture screw out some in the meantime, but that's not a fix, just a bandaid and probably won't entirely fix the problem.

You can also confirm this by riding a little with the choke partially open. That will richen things up and should make the popping go away.

Let us know how those steps pan out and if that doesn't fix it, we can go from there.
OP
@xperimental21 avatar
UTC

Hooked
Super
Joined: UTC
Posts: 100
Location: -7 degrees
 
Hooked
@xperimental21 avatar
Super
Joined: UTC
Posts: 100
Location: -7 degrees
UTC quote
chandlerman wrote:
Usually, that popping and hunting is indicative of a too-lean idle jet.

Is this a new thing, or has it always been like that. If it's new, start by cleaning the idle jet. If that doesn't fix it, you may have picked up an air leak somewhere. If it might be an air leak, don't ride it hard until you can get it sorted, as that's a great way to seize a bike, especially coming off the throttle if you're already lean.

If it's not, then you need to get a richer idle jet. Try a 52-120, maybe grab one either side of that, and it should sort you out. You can also try taking the mixture screw out some in the meantime, but that's not a fix, just a bandaid and probably won't entirely fix the problem.

You can also confirm this by riding a little with the choke partially open. That will richen things up and should make the popping go away.

Let us know how those steps pan out and if that doesn't fix it, we can go from there.
It's been like that since I owned the scooter.

I closed the exhaust outlet to check if there was air leak, the idle went stall and died, so I am sure there was no air leak. The jets were clean, not dirt clogged.

Anyway, I was riding, closed the throttle, pulled the choke partially, then the revs came down nice and smooth! no popping.

Is the 42 without air corrector not rich enough?, sorry i didn't know about that. I tried to screw out the mixture screw in hope it would get richer (more mixture) with the 42 and 42-140 idle jet. The spark plug color showed almost sooty black but still popping.
UTC

Jet Eye Master
PX221 MHR, O tuned PX200, PX125 and some motorbikes
Joined: UTC
Posts: 4568
Location: London UK
 
Jet Eye Master
PX221 MHR, O tuned PX200, PX125 and some motorbikes
Joined: UTC
Posts: 4568
Location: London UK
UTC quote
I suspect your carb has a built in air corrector and needs the pilot jet without a hole.
Popping on rundown means lean.
Post a picture of your carb to confirm.
OP
@xperimental21 avatar
UTC

Hooked
Super
Joined: UTC
Posts: 100
Location: -7 degrees
 
Hooked
@xperimental21 avatar
Super
Joined: UTC
Posts: 100
Location: -7 degrees
UTC quote
Jack221 wrote:
I suspect your carb has a built in air corrector and needs the pilot jet without a hole.
Popping on rundown means lean.
Post a picture of your carb to confirm.
Is this one?
Forum member supplied image with no explanatory text
Forum member supplied image with no explanatory text
OP
@xperimental21 avatar
UTC

Hooked
Super
Joined: UTC
Posts: 100
Location: -7 degrees
 
Hooked
@xperimental21 avatar
Super
Joined: UTC
Posts: 100
Location: -7 degrees
UTC quote
I also realized that some part of the air filter was wet, it looked like the fuel was spitting out through the intake.

So I pulled the carb and do some checking on the rotary valve, the spark plug was removed, then I put some fuel when the rotary valve was opened. Kickstarted few times and some fuel was spitting out through the intake and nothing spitted out from spark plug hole, see video below.

External inline image provided by member with no explanatory text
https://www.youtube.com/shorts/CBxUIjqzfbc

Then I put the spark plug and tighten it. I put some fuel when the rotary was closed, it looked like the fuel sucked slowly with bubbles?, could be air from inside?

External inline image provided by member with no explanatory text
https://www.youtube.com/shorts/bVluo_R2WHo

Is that normal?
@swiss1939 avatar
UTC

Ossessionato
P208, Stella VMC Stelvio 187, Stella 150, VNX1T, V9A1T, V9B1T, 02 Sportster XLH1208
Joined: UTC
Posts: 4110
Location: Staten Island, NY
 
Ossessionato
@swiss1939 avatar
P208, Stella VMC Stelvio 187, Stella 150, VNX1T, V9A1T, V9B1T, 02 Sportster XLH1208
Joined: UTC
Posts: 4110
Location: Staten Island, NY
UTC quote
xperimental21 wrote:
I also realized that some part of the air filter was wet, it looked like the fuel was spitting out through the intake.

So I pulled the carb and do some checking on the rotary valve, the spark plug was removed, then I put some fuel when the rotary valve was opened. Kickstarted few times and some fuel was spitting out through the intake and nothing spitted out from spark plug hole, see video below.

External inline image provided by member with no explanatory text
https://www.youtube.com/shorts/CBxUIjqzfbc

Then I put the spark plug and tighten it. I put some fuel when the rotary was closed, it looked like the fuel sucked slowly with bubbles?, could be air from inside?

External inline image provided by member with no explanatory text
https://www.youtube.com/shorts/bVluo_R2WHo

Is that normal?
Measure the rotary valve clearance on both the front and the back with feeler gauges and report back what you get.
UTC

Jet Eye Master
PX221 MHR, O tuned PX200, PX125 and some motorbikes
Joined: UTC
Posts: 4568
Location: London UK
 
Jet Eye Master
PX221 MHR, O tuned PX200, PX125 and some motorbikes
Joined: UTC
Posts: 4568
Location: London UK
UTC quote
xperimental21 wrote:
Is this one?
Yes, your carb must use the pilot jet without a hole. 42 should be ok but may need bigger.

If a rotary inlet holds fuel mix for more than 10 seconds there's not much wrong with it
OP
@xperimental21 avatar
UTC

Hooked
Super
Joined: UTC
Posts: 100
Location: -7 degrees
 
Hooked
@xperimental21 avatar
Super
Joined: UTC
Posts: 100
Location: -7 degrees
UTC quote
Jack221 wrote:
Yes, your carb must use the pilot jet without a hole. 42 should be ok but may need bigger.

If a rotary inlet holds fuel mix for more than 10 seconds there's not much wrong with it
yes thank you. Anyway, need advice. This is the result on WOT, Is it lean and need to up the main jet or I need colder spark plug?

I ride mostly 30 km, average weather temperature is 30 deg. celcius, I use B7HS plug.
Forum member supplied image with no explanatory text
Forum member supplied image with no explanatory text
UTC

Jet Eye Master
PX221 MHR, O tuned PX200, PX125 and some motorbikes
Joined: UTC
Posts: 4568
Location: London UK
 
Jet Eye Master
PX221 MHR, O tuned PX200, PX125 and some motorbikes
Joined: UTC
Posts: 4568
Location: London UK
UTC quote
Up the main jet. Increase the jet size until WOT doesn't rev out, then reduce jet size until it just about revs out. A jet that revs highest is too lean on the road.
OP
@xperimental21 avatar
UTC

Hooked
Super
Joined: UTC
Posts: 100
Location: -7 degrees
 
Hooked
@xperimental21 avatar
Super
Joined: UTC
Posts: 100
Location: -7 degrees
UTC quote
Jack221 wrote:
Up the main jet. Increase the jet size until WOT doesn't rev out, then reduce jet size until it just about revs out. A jet that revs highest is too lean on the road.
Done! feels faster and great now.

Anyway, should the timing need to be advanced a bit nowadays? I still use stock [P/] 150 2-ports cylinder (it is actually 58.6 mm bore, 57 stroke), old style cylinder head without squish chamber (chamber shape like an inner dome) and 90 octane fuel. I still use 22 degree BTDC and contact breaker.
UTC

Jet Eye Master
PX221 MHR, O tuned PX200, PX125 and some motorbikes
Joined: UTC
Posts: 4568
Location: London UK
 
Jet Eye Master
PX221 MHR, O tuned PX200, PX125 and some motorbikes
Joined: UTC
Posts: 4568
Location: London UK
UTC quote
Where did the jetting end up? Going faster is a good sign. When the jetting is good, if you roll off the throttle, then open up again the power should come back Immediately.

Points ignition is not much good for higher rpm. Leave the timing at 22. Plenty of scope for upgrading. PX150 head and vape conversion top of the list.
OP
@xperimental21 avatar
UTC

Hooked
Super
Joined: UTC
Posts: 100
Location: -7 degrees
 
Hooked
@xperimental21 avatar
Super
Joined: UTC
Posts: 100
Location: -7 degrees
UTC quote
Jack221 wrote:
Where did the jetting end up? Going faster is a good sign. When the jetting is good, if you roll off the throttle, then open up again the power should come back Immediately.

Points ignition is not much good for higher rpm. Leave the timing at 22. Plenty of scope for upgrading. PX150 head and vape conversion top of the list.
110 MJ is the best so far. I tried to up the idle jet from 42, but only 50 and 48 sizes were available for my old carb. Unfortunately, the idle jets were too rich, it kept idling when the mixture screw fully closed/tighten and too much smoke.
DoubleGood Design banner

Modern Vespa is the premier site for modern Vespa and Piaggio scooters. Vespa GTS300, GTS250, GTV, GT200, LX150, LXS, ET4, ET2, MP3, Fuoco, Elettrica and more.

Buy Me A Coffee
 

Shop on Amazon with Modern Vespa

Modern Vespa is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com


All Content Copyright 2005-2024 by Modern Vespa.
All Rights Reserved.


[ Time: 0.0719s ][ Queries: 4 (0.0384s) ][ live ][ 313 ][ ThingOne ]