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Hi, I have a (new to me) wonderful 180 SS that accelerates pretty well through the gears, but stutters really bad while coasting.
Mostly in 3rd and 4th. The bike shakes and shudders quite a bit. If I just give a tiny bit of throttle, it's fine again - I even have to apply the rear break a little to normally go in slower traffic.
I've searched this forum for similar situations, but was not able to find any.
I'm quite new to this community (and to owning a classic Vespa) - any tips on where to start?
Thanks a bunch!
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lean mid throttle?
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Thanks for the response, appreciate it. Thing is, I do not understand all things (like lean mid throttle) yet, it happens when the throttle is fully closed. So it's tinkering around with the carburettor mostly? It's got a SI 27/23 with a 160 main jet (I think). It also has a P200 piston - so it's more like 225CC, if that helps? Thanks for your patience!
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Hi.
Can you replicate the shuddering in neutral just by giving it a lot of throttle? If so, can you post a video of this shake/shudder?
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It only stutters while driving without any throttle. Anything with throttle goes smooth (ish). I'll try and get some sound/footage- maybe that helps indeed.
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Here's an example of the stuttering while coasting. Any thoughts or ideas are greatly appreciated!
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that looks more like a misfire. does it tickover?
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Sorry, had to find what tickover means - it idles fine, I think. No stalling or strange sounds, if that's what you mean?
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Yes, thats what i meant.

My thinking was that if it did't idle, would that cause the symptoms you are seeing. eg, blocked pilot jet, and underfueling as you come off the throttle?
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Molto Verboso
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Yes, it sounds like the idle circuit/ mix is off. Once the throttle slide is opened it's running fine on the main but starving at idle.

I also wonder if a P200 piston is right for the piston-ported SS cylinder.
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Thanks so much for the pointers so far, it gives me a place to start! Really appreciate it. I read up more about how to adjust the carburetor mix settings, still learning how to do that properly... now I'm doing it a bit by ear, with the idle jet out 2,5 turns and the mixer screw about 1,5 as a starting point. But indeed, since the piston is not standard, it's all a bit of trial and error now. I find the sparkplug also quite dirty and wet, so that might also indicate something.
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I think you do need to figure out how to set the idle mix. There is the throttle stop screw ( the vertical one towards the front) to set the speed and the mixture screw at the rear. It may be idling now on the main. I am not sure what you mean by the idle jet being 2.5 turns out. The jets are not adjustable. I'd take them out in any case and make sure they are clean. Might help to give the entire carb a good cleaning.
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Sorry my bad, I meant idle adjusting screw and mix screw. Still learning! This community has already been super helpful with learning resources - I found a blow up of the 27.23 carburetor, so I'll go try some of that and will keep you posted!
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Different carb, same basic procedure :
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Careful riding.
That could damage the motor.
Sounds to me like you are starving for fuel when off the throttle.
Assuming no modifications - I suspect one of two things.
1. Your idle mix screw is turned all the way in or nearly.
2. There is junk in your idle circuit - so it is starving.


If it were me:
- I would check my idle screw by first turning it clockwise until it stops (careful - the material is soft - don't over tighten), count the turns. When you unscrew it again, unscrew it about 2.5 turns. (Let us know where it was set)
- If you find it is already about 2+ turns out, I would pull the idle jet and blow out that circuit with some compressed air. Hold a rag over it - fuel will squirt out back at you when you do this. Best to have the idle screw removed when you do it if you can - to allow junk to come out - and to blow it out from the idle screw hole backwards as well.

Have you made any other recent changes?
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I'm only doing small test rides now, so no longer drives. And I just checked - Idle screw is about 2,5 turns out indeed. I believe all jets were replaced last time I brought it in for service (I haven't had it for long - so no reference points there, and it was not running nicely when I got it, and not after service...)

I tried to set the mixture/idle like in the previous youtube movie and with some other guides I found here, so that should be roughly in the neighborhood, but not great. It is still stuttering while coasting in the meantime.

I'll see what happens when I use some compressed air. The choke also doesn't seem to do anything - could that be a sign of something?
thanks,
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Molto Verboso
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The choke ( actually a fuel enrichment valve) adds fuel and also runs through the idle circuit. Pulling the knob should make it run badly : too rich. Mixture screw should be about 1.5 out.
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Wonder if with the stock carb and jets if the pilot circuit can provide sufficiently with the bigger bore?
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I'm still curious if the P200 piston is right for the cylinder, though it seems to run fine above idle.... ?
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Is it lean, or overly rich especially with the wet spark plug? Could the surging be an overly rich mixture not firing or firing at the wrong times in the exhaust then when you open the throttle and get a bit of air in it fires normally? Maybe try and turn the idle screw for rpm as low as possible so it wont idle and the slide is closed all the way and try it again. This should be even less air and the problem could stop or get worse, wondering about worse as there is an air bypass.
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Thanks so much for thinking along all! I just blew out the idle, doesn't seem to have helped much, but what I did discover is that the idle screw doesn't do much after about 4 turns - can't get the idle to slow down much. After a blip of throttle it takes quite some time to get back to idling. Mix is about 1.5 turns out. On the bigger piston - that supposedly worked great 8-10 years ago, but that's quite some time. The main jet is 160-BE3-135 if that helps. I'm going to take the carburetor off and thorough clean it at home, see if that makes a difference. The idle direction does make a lot of sense ... to be continued...
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No, as far as I understood from the builder, it was a third over p200 piston and piston ported. (Probably 8-10 years ago) did 25hp on a dyno at the time, so definitely not stock. That makes finding the right settings/troubleshooting a bit difficult as well.

Cleaning the carburetor right now, ordered some replacement parts (slide looks like it is damaged a bit on the side with scuffmarks) and then I'll put it back together again and see where we are
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Christopher_55934 wrote:
Is it lean, or overly rich especially with the wet spark plug? Could the surging be an overly rich mixture not firing or firing at the wrong times in the exhaust then when you open the throttle and get a bit of air in it fires normally? Maybe try and turn the idle screw for rpm as low as possible so it wont idle and the slide is closed all the way and try it again. This should be even less air and the problem could stop or get worse, wondering about worse as there is an air bypass.
I noticed the slide was also not moving all that much with the adjustment screw, so I'll see if I can make that go a bit smoother as well.
Fun things, these vespas!
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The slide looks like it is wearing mostly on opposite corners with little or no wear on the others. Maybe the carb body is warped ?
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Hope not... is that even possible? And how can one check?
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How does the slide feel if you just slide it in and out by hand ? That's a pretty solid base on those carbs with three mounting points. People do stupid things like add another gasket to "seal it better" and over-tighten them so they compress and warp the flange.
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It slides super easy without any resistance, so I'm assuming that might be a very old slide... I'll get the carburetor back together again by next week and go from there again. Thanks again for all the great tips and help!
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Quote:
After a blip of throttle it takes quite some time to get back to idling
When I posted previously - I did not realize the bike had been tuned and rejected. Suspect it needs a bit of finessing with the jetting. This is a certain sign you are lean.

It would be helpful if you post all the details in one post.
Exhaust
Cylinder
Carb (27/23? not sure I've heard of that)
Main jet stack - Air corrector at the top, atomizer in the middle, and main jet.
Idle jet.

The P200 piston would keep it a 200CC.
Is there an aftermarket cylinder or a stock one on it?

-CM
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Let me try with the information that I managed to gather. And again, thanks a bunch for all your patience and help! It's not easy, diagnosing while you don't really know what has been done to the bike!

I bought it last year, so it's all been by discovery or information I received from the guy who put it together originally, some 8 years ago. It has sat at a dealership for at least 3 years before I bought it.

Exhaust: (50 year old) Ken Cobbing sports exhaust
Cylinder: it does have a third over P 200 piston in it which he welded shut and made it Piston Ported. With the bore and the stroke it's probably 227 ml (info from the original builder)
Carb: Original Dell'Orto 27.23 without modifications
Main Jet Stack : 160 BE3 135
Idle Jet : 50
Choke jet : 60
Spark plugs : NGK B7HS

I ordered a set of new jets and a new slide in the same sizes, just to be sure it's all clean and new, to at least establish some sort of baseline to work from, because I do understand I shouldn't be changing too much at the same time to get it running smoothly again. Plug seems to get dirty and wet quite quickly, so it might be too rich. On the other hand, no issues with bogging on the throttle through the gears.
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The 180SS is more like a GS than anything else. Is piston ported intake, like a lambretta and also a stock 60mm crank, like the prototype Rally's.
Boring the 61mm SS bore for a 66.9mm P200 piston would make a fair difference but not change 10bhp into 25bhp on its own.
The SI 27/23 is an old style carb and there probably lies the issue. This carb uses a pilot jet with a hole in the tip but no air hole. By the sound of it running, I suspect this pilot is either, very blocked in the tip (hard to see or clean) or has been changed at some point for the newer (incorrect for this carb) pilot jet with the air hole in the top.
If this scooter is clean and original looking it's worth a fortune.
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Well - now you have the expert on it ^
So I can erase what I was typing...
How about a pic of the idle jet to see if it has the hole in the top?
If not - one trick for cleaning can be to take some stranded copper wire - and unstrand it so you have a single strand or two - and use these stands for a pic.
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I can't tell you enough how much I appreciate all your help! It's really nice that you are willing to help a newbie with this much patience!

From what I can see, no hole in the top of the idle jet (top 2)

And Jack221 - it sure is clean and original looking, and already cost me a fortune 😂... But it's worth it to keep these classic scooters going, if you ask me
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Well good. Has the correct type of pilot jet. Last chance for an easy fix, is the tip blocked?

If actually 25bhp a 50 jet size is too small for that carb but a small jet still wouldn't run that bad.

There is a remote pilot air hole near the jet but I think under the Venturi cover. Find it and check that is clear through from the top via the jet to the mixture screw and into the holes under the slide (there are 2 outlet holes) and beneath the slide.
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The 25HP was dyno confirmed at the time of finished building, according to the guy who restored it, but I havent seen proof, so that could be wildly exaggerated of course. I'll go check the carb housing and there seems to be the tiniest hole in the bottom of the idle jet.
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Hi, With your pilot jet, if its anything like the smallframe carbs, there should be a jet hole at the other end. ie bottom rather than top.
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One thing to think about, this maybe how it ran back then. Previous owner may have always been on the throttle and not cared about slow riding street manners. Where your going slow he may have dropped a gear and gave more gas. Where your coasting with motor, previous owner may have disengaged clutch and idled.
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The specs for a stock P200 are shown as 12.5 hp.
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Great info Jack do you think the 135 main jet might be contributing to the wet plug?
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Quote:
... it sure is clean and original looking, and already cost me a fortune 😂...
Oh my. That is an outstanding Grimstead repro.

What exhaust do you have and how old is it?*. Sounds like it might be partially blocked.

* edit: just re-read... 50 yrs old?
⚠️ Last edited by SoCalGuy on UTC; edited 2 times
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