Modern Vespa

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Hi all

I'm trying to diagnose a problem I have with my GT125, its a 2006 model with 20K miles on the clock. (Edit - I've recently fitted a Scorpion Titanium exhaust, and lightened the weight of the rollers to allow it to rev to its power band)

When accelerating the scooter is lurching as if the clutch is slipping, but I'm not too sure whether its clutch issue or a carb/engine type of problem. The engine's revs don't rise like a car's would so I'm not convinced its the clutch, plus i took a look at the clutch and cleaned it up and it seemed ok.

What happens is that at low revs its ok, at top speed its ok, but when accelerting mid range it stumbles a bit/lurches.

My thoughts have led me to think that its one of these, but I'm struggling to diagnose further and wondered if you would mind helping me please....

1) Clutch slipping - but it looked ok when i cleaned it up recently
2) Carb setting/cleanliness - is it possibly bogging at mid-range, needle settings perhaps?
3) Valve adjustment - but the scooter starts ok whether hot or cold so I'm leaning away from this

....or something else maybe....

I hope I've described the symptoms sufficiently for you to help me, and any and all input would be fantastic, please chirp up!!

Fabio

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could be a glazed belt, or a lot of gunk on the variator, or a groove worn into the variator. My other bet would be a loose spark connection.

+1 on the loose spark connection. Had this happen to me this past Sunday.

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I thought it may have been the spark plug too, I removed it last night and checked it and re-tightened the plug, I also cleaned out the spark plug cap and made sure it was a good tight fit...also to no avail.

I've got the old plug somewhere I might give that a try to see if its any different.

Any other thoughts??????

Fabio

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I've investigated this a bit more now....

I'm thinking that this is clutch/CVT related, as I'm now experiencing a slipping noise from stationary from the rear pulley area....and also a little 'bunny-hop' at take off!!

Fabio

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Maybe the belt is starting to go? When I had my lx50, it started to feel like the belt was slipping like a thousand miles before the belt finally broke.

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Its a brand new belt, only fitted this week....

I've found the problem. I investigated a bit more and took the CVT cover off again....the inside of the clutch bell had a light smearing of grease on it (lord knows where that came from, hehe) I did grease some of the CVT components lightly and some must have transferred to the clutch somehow.

So... all sorted!!

Thanks everyone for your help with this - much appreciated.

Fabio

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Fabio...had a similar problem with "stuttering" at the rear wheel and "flat spot" when rolling on the throttle. My solution was easier...I pulled the CV Carb cover (4 screws), pulled out the spring and retainer, diaphragm and slide/needle and put 3 #4 stainless washers washers under the needle, cleaned the guillotine slide with rag soaked in solvent (being careful not to get it on the diaphragm). Reassembled and no more stuttering or flat spot (same trick I did on my Harley CV carb which is similar). Problem solved... 8)

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Hi RW

Yes I've been contemplating some washers as shims under the needle....

I was discussing this with Gazza in another thread>>> GT125 + Scorpion Exhaust (Post 1149671) So its interesting that someone else has had this problem too and has solved it with the same thoughts as me!!

Much appreciated post!!!

Fabio

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Aaaaaargh !!!! Its come back again!!

Its not clutch related - its more like carb related... and also now sometimes at idle it cuts out completely without warning.

When its running at low revs or at WOT, it runs well, its when I change acceleration it stumbles as if there is a fuel supply issue perhaps? Then again under exactly the same conditions a moment later it runs really well, and then perhaps a mile or so later it blips as if its being starved for a split second.

At idle sometime if I give it just a bit of throttle it stumbles, and at other times its fine (hot or cold doesn't seem to make any difference)

So I took the carb off and completely cleaned it with carb cleaner, the throttle body, venturi mouth, butterfly.... all the little passageways, the jets both main and idle, the accelerator pump, and so on and so on.....

But its still the same.....


Has anyone got any ideas because its doing my fruit in now!!!

Fabio

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From your posts in this thread and others, you appear to be very savvy with troubleshooting but If you haven't checked these things already, take a look at them.

*Does your carb have a diaphragm? If so, you have inspected it for cracks?

*If your float is hanging up as it goes up and down in the bowl, it can do this. Inspect your float for jerky movement/rough areas on the pivot.

*Inspect your fuel lines for kinks.

*Inspect the hose that goes to the vacuum port on the carb.

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Hi XLR8....

...and thank you so much for your input...

I had half thought about the float in the carb being the problem perhaps but the points that you have suggested make a whole load of sense and they will be my next attentions, particularly...

*If your float is hanging up as it goes up and down in the bowl, it can do this. Inspect your float for jerky movement/rough areas on the pivot.

This I'm guessing will perhaps starve the carb of fuel for split seconds, yes???

Thanks once again!!!

Fabio.

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Yes..............it can happen if the tang is hanging on the pivot pin on any carburetor. Not enough gas in the bowl to keep an even flow so it sucks some air. I'm not positive that this is your problem but it's worth a look just to make sure.

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Thank you again XLR8...

While I'm messing with the carb, I'll give the diaphragm a good inspection too !!

Fabio

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BTW: I'm not familiar with this specific carb...............but, does it have an accelerator pump mounted on the body of the carb? If so, that's a big, red flag to look for. If it doesn't, never mind.

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Yes it does - its a Keihin CVK - tell me why??!!??!!

Fabio

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The accelerator pump shoots a shot of extra gas when you twist the throttle to keep an adequate supply of gas in the carb so if the pump is malfunctioning, it will cause a stumble.

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When i cleaned the carb it was stiff, so I took the pump apart and got it pumping again.

I might just revisit that too to make sure its super-smooth.

Fabio

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Hi again

So tonight's continuing saga of the GT125 "nightmare" hehe (the truth is I'm quite enjoying taking the carb apart and getting to understand how it works at bit better, its different to both of the 2T carbs that I've had on my ET2.

So.... I took the carb off again and inspected the diaphragm, float, float needle and float hinge, all fuel pipes and vacuum pipes...also took the accelerator pump apart and made sure it was clean and pressed it manually to ensure it was pumping fuel (which it definitely was).

I also cleaned out the main and idle jets again, the main looked fine, the idle im not so sure.

I've tried to answer in the same paragraph order so that it makes sense...

1) Does your carb have a diaphragm? If so, you have inspected it for cracks? Its clean and in good condition, no cracks or splits

2) If your float is hanging up as it goes up and down in the bowl, it can do this. Inspect your float for jerky movement/rough areas on the pivot. Inspected the float and its smooth and functioning as i'd expect

3) Inspect your fuel lines for kinks. Had a look at those...All in good condition

4) Inspect the hose that goes to the vacuum port on the carb. In good condition, no splits or cracks

I put the carb all back together and it was a very similar story.... the scooter runs well for about 30 mins, accelerates well, idles well, all felt brilliant....and then I came to a junction (one of many, it just wasn't the first one) and at standstill the scooter chugs and dies. I start it again and its starts, but then I struggle at each junction to keep it running, but it runs well when not idling.... but then five minutes later it starts to stumble mid range through the acceleration too.

Here are my thoughts.....the carb is now spotlessly clean, the only area where I wasn't too sure was the idle jet, and even though it been cleaned out (I could see a perfect circle when put to the light last night when I cleaned it and also again over the weekend too) it wasn't clean when I took it apart and only from yesterday.

I've read somewhere (or think that I have) that the idle jet/atomiser goes bad sometimes - I think I read this in a post from cheekythomas once (but I may be wrong there!).

Other thoughts....

- Fuel tap/pump?
- spark plug/HT lead?

I'm leaning towards the idle jet/atomiser going bad but for the life of me I can't understand why it doesn't stay perfectly clean for such a short period of time, but I could just be clutching at straws with this maybe????

Any thoughts ?

Fabio

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We don't have your model in the USA. Based on what you've already checked, here's my thoughts:
*As inexpensive as they are, you may want to change out the idle jet just to see if that's the problem.
*Does it have a fuel pump like the GT200? If so, it's certainly a possibility that it's acting up when it gets hot and working okay when it cools down.
*I once owned a motorcycle that had an erratic coil. Possible.
*Definitely make sure that the HT lead is tight on the spark plug cap and that the cap is firmly seated.
*Doesn't sound like a bad fuel tap since it works for X period and then stops and then works again.
*An erratic CDI that acts up when it gets hot.
*If your gas tank is building up pressure because the vent line to the tank is kinked, it could also cause this. It's worth a look just to see.
Hope you find it quickly.

I pretty much agree completely with XLR8, with the exception that I think there is a slight possibility that it could be the fuel tap (if the GT125 has one). If there was a slight vacuum leak in the line from the intake manifold to the tap, I would think it might be theoretically possible that some fuel is getting through, but not enough to meet high demands. On the other hand, you say the problem often manifests itself at idle. Hmm.

I'm starting to think it might be a heat-sensitive ignition part, like the coil. I think the whole HT circuit (coil, wire, and plug cap) are one piece on all the Vespas (at least they are on mine), and relatively inexpensive. Might be worthwhile to swap it out as a test. If it proves not to be the problem, at least you have a spare for a part that fails fairly often.

How long has this been going on? Could it be possible that you just got one bad tank of gas?

Check the spark plug boot to make sure the contact is screwed all the way down into it. I had this issue recently.

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I think I'm going to start with the spark plug/cap, that's one area that everyone seems to mention as being the problem....I've had a look at it before, but I think that I'll take it off the scooter completely (or at least loosen it from the body/frame) and give it a good checkover and cleaning... I've never thought of that - does the connection/contact work loose??
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And if that doesn't solve it then I'll move onto... And if that still doesn't do it then I'll move onto the fuel filter/fuel tap/fuel pump, I haven't got as far as checking if these are all (duh!) fitted to my scoot but I'm assuming that they are - is there any process for determining which of these may be the culprit ???
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. The other thing that keeps going through my mind is when this problem/s started, I can't recall exactly when....but I remember swapping my spark plug for a new one and then the problems started sometime after that (I wonder if the new spark plug is the problem and its just duff). I've still got the old one so I might try that one back in.

If not then the one thing I just don't understand is why I've suddenly got 2 problems which don't seem related, there must be a link somewhere between the 2 probs I'm experiencing, or am I just over-thinking this and I've just got 2 problems?

Fabio

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As it is intermittent I think it unlikely to be a blocked jet. The overflow from the tank comes down a black rubber hose to the bottom of the bike. Check a vacuum is not being created by undoing the fuel cap and listening for a hiss of incoming air. The fuel tap is unlikely to cause intermittent problems and normally jam open.

The next step is the HT CDI system...

I am no mechanic by all means but the fact you started the trouble after having changed the spark plug rings a bell.....
Good luck and keep us posted

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Yes I hope its that simple....I've already had a look the plug once and it all seemed ok, but I'm going to have another good look at it tonight.

Thanks JackR - much appreciated!!!

Fabio

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DougL has mentioned another potential thing to look at.... the fuel pump...

I think I remember reading somewhere that there was a batch of dodgy fuel pumps, I thought it didn't extend to the 125, but on the parts diagram it says "Fuel Pump Leader 12/180 CM120901" so I'm not too sure.

Anyone know this?....or indeed how to check whether the pumps is faulty or not?

Fabio

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Ok - so an update on tonight's diagnostics activities....


I removed the new spark plug and replaced it with the old one, and at the same time removed and checked the HT Lead and Plug Cap, and also had a look at the connection to the coil.

All seemed ok, I cleaned everything up and tightened the spark plug connection inside the cap (it was already tight).

Still no joy, I went out for a run for 20-30 mins with no major problems....then after a blast along a dual carriageway at 60mph, the very next junction (exit on the slip road) it stalled, it chugged first and then when i applied a spot of throttle from idle and the idle went 'lumpy' and even by holding the throttle still or trying little "revs" it just petered out, It started again immediately as it always does but then I had to keep it on the rev a touch to stop it dying. The mid-range throttle was present too but only on acceleration not at constant speed. Hmmmm, I'm thinking fuel supply is the weak link - am I right here??

I checked to see what was powering the fuel supply on the scooter, its as I thought...fuel tap, fuel pump, fuel filter. I'm guessing the one to tackle first is the pump - is this disassemble-able, or is it a "buy new" type part

Fabio

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Sounds like the fuel pump or at least fuel supply indeed
Sorry no further help from me but I am sure others will chip in.
Good luck

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Ok ok ok, so here's the exciting news....

Its definitely 2 separate issues.... I've solved one....and I think I've got the answer for the other!!!

As you will know...when I went out to test what I'd done with swapping the plugs last night, I was very disappointed when it seemed to make absolutely no difference, the idle seemed to be getting worse as the scooter was warming up. The last thing I noticed when coming to a stop and trying to keep the revs on whilst negotiating the scooter into the garage at home was a misfire.

I thought nothing more of it, but then at work I started to think...what if it was 2 separate problems that i was facing after all, one being spark related (the idling problem) and the other being fuel supply starvation of some sort (the mid-range stumbling).

So I thought that if I tackled the spark plug side of things first then at least I could get the scooter running without it constantly cutting out.

So I decided that because the spark plug I'd fitted was brand new (only this month) then that should (in theory) not be the fault. So I decided I would try to diagnose my idle/spark problem by progressing onto the next step in the "spark chain".... testing and swapping, and then onto the next item until I found what was wrong.

I enlisted the help of a colleague at work to try to find a spark plug cap at short notice as I wanted to get it up and running as soon as I could. I work as the Finance Director of a motor dealership so I asked the parts manager if he could source me one from one of his suppliers, I had forgotten to make a note of the part number like a fool... so i sent him off on a bit of a wild goose chase (unfair of me I know!).

Anyway, he returned fairly quickly with a choice of two on a Sale-or-Return basis, "Just bring me back the one you don't fit and I'll get a refund for it".

So when I got home tonight I removed the old one and fitted the VB05F plug cap and then refitted the new spark plug...and then went for a run.

I scooted for over an hour, and experienced not one idling problem, neither while hot nor cold, neither after running flat-out or "pootling", so to say I'm chuffed would be an understatement.

Total cost of the repair.... £2.60.... Yay!!




So...onto problem number 2....

Assuming that that the problem with the mid-range stutter was now a fuel supply issue of some sort, I started to re-read the posts that you all had left me in this thread and also in the the other thread>>> GT125 + Scorpion Exhaust and I tried to eliminate in my mind which it may now be and which it definitely wasn't. With sorting out the idle problem I was now fairly certain that spark related theories were now not right.

Three posts made a lot of sense, now that I was looking at them in a different light...

1) One was about fuel pumps but my thoughts are that a fuel pump failing would be across the throttle range and not just mid range.
2) The second post that I focused on was about spark plugs but more in the sense of "that's what you last messed with before it broke" and that made a lot of sense so I've considered what else I've messed with and I'm thinking that the Scorpion exhaust may have something to do with it.
3) Thirdly was the post about mid-range flat spots and shimming the carb needle with washers to make it run more rich mid-throttle, and I figured that the Scorpion may need that.

So what I did was to remove the needle and shim it with 3 small stainless washers as suggested and take it for a run out again (about 30mins).

What I did notice was that the idle was still good throughout the whole run, but even more interestingly was that the stuttering was more apparent at 3/4 to WOT, it had moved up the range from mid-throttle!!

I'm thinking that I need to change the main jet up a size or two perhaps to "feed" the new exhaust which I'm guessing is being a bit starved, I havent had chance to plug chop with the other problems going on.

What are your thoughts??

Fabio

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I've just updated the title of this thread to "GT125 stumbling/lurching on accel/idle - Partially solved!!" for anyone reading it in the future so they will know what I did to solve the first of these problems.

I'm going to try a few different Main Jet sizes - I'm guessing that I should go up by perhaps 5-10% to cover the extra thirst of the Scorpion, its currently a 105/106MJ ( I can't quite make out what its says on the side of the Jet but I think its a 105).

So I guess I'm looking for a 110 to 115 MJ size?

Any ideas here?

Fabio

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Fabio,

Thanks and good luck with the Main Jet sizes

Do you think I will also possibly have the same 'problem' after I have my Leo Vince Installed ?

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I'm guessing that both Scorpion and LV will say that their exhausts will run ok without re-jetting or changing rollers.....and they probably will on most occasions.

...but to get the most out of them I feel that you will have to do both......especially if your scoot is a 125/150 as opposed to a 200/250

.... I've already lightened my rollers see this thread>>>> GT125 + Scorpion Exhaust

... and I'm just going to try a larger jet (If I can find out which one is the right size)

Fabio

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Fabio,
excellent thread as usual, it's great to see your thought process mapped out on screen. Can't offer any useful advice i'm afraid as i'm not technically up to speed YET, but getting there slowly. I'm Putting everything back together tonight with the new pollini clutch and DR pulleys rollers, so fingers crossed !!
Good spot with the spark plug cover, i'll be looking at that next if my starting problems persist.

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Hi Inch...

In all honesty...its a lot easier to lay it all out (my thoughts are that is) because it puts it all into some sort of order for me.

It makes it also very easy to refer to later on to remind me what I did and in what order, I also tend to take a lot of photos as I do things to give me a kind of "aide-memoire" for later.

Also by laying all my thoughts out it helps others to come in with their thoughts (positively encourages them )

And finally, by noting everything down it also helps others with similar problems

Fabio

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Fabio,
apologies for hijacking your thread here as I know you are up to your neck in it but perhaps you could answer a quick question as I know you have messed about with your rollers- (fnar, fnar)
I have just fitted the 6 new Dr pulleys rollers into the variator, and the 3 sliding pieces on the variator back plate. However, with the 3 new sliding pieces fitted, the variator motion of the back plate moving in and out of the variator, feels very stiff, not excessively so but much stiffer than with the stock sliding pieces fitted. When I turn the variator over so the shiny side is facing up, the back plate does not move of it's own devices. When I have the stock sliding pieces in place, the variator is quite free to move in an out, and when i turn it upside down the back plate falls on it's own. Do they loosen out ? they appear to be the correct size and correct part number as per the DR Pulleys website.
Am I overreacting? should I just lash the variator back on and take the bike for an extended burn tonight?
thanks !!

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I'm not too sure, I haven't got much experience with Dr Pulley sliders.

Someone else will have to chip in with an answer here....or perhaps start a new thread oops just seen your new thread >>> DR Pulleys Question

Fabio

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I did not realize that you were not running a stock exhaust or I would have mentioned that you may have to re-jet your carb to make it run smoothly through the entire rpm range. I'm happy to hear that you've solved at least part of the problem and I think that you've actually found the other part with the jetting.

No worries at all, thanks for the reply Fabio. Again apologies for jumping in on your great thread. I'll just observe from now on
I'll wait and see if someone replies to my thread who has fitted the DR Pulleys sliders .
I was getting impatient and you are my "Go To Guy" for technical issues,
thanks,
Frank

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Hi XLR8

Yes its a Scorpion - sorry I thought that I'd said

Does my theory on it requiring a larger MJ make sense, do you think? It seems to carry a lot of logic in mind, I know from other members on the site that they don't bother up-jetting when they fit a freer flowing exhaust..... but to my way of thinking that just doesn't make any sense, and the gains that are available (perhaps only slight) aren't realised.

Fabio

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