Modern Vespa

Morning all,

Summary of the story below - scooter doesn't have low end power now. It idles well, dies when I rev it too hard, I have to "pump" the throttle to build revs and then at some point with high revs I can get it to pull away, and it will really pull away fast and then die. When I mean fast, front wheel can lift a bit which never happend before i changed the cylinder. Anyway, I changed a load of things to get to this point and I need some help to determine how to get her running well.

So, last year I had some issues with getting my p125x running correctly. I had installed a new cylinder that I bought from a reputable German dealer, but things still were not quite right and I couldn't tune the carb. I had a few issues trying to seal the exhaust too which i think contributed to the problem. Another thing was that I had a very spongy kick start feeling that just felt off and I think it affected starting ability. so I had the following:

1) carb tuning difficulties - lack of power
2) leaky exhaust
3) spongy kickstart

Over winter I pulled out the engine. I took off the cylinder and found that the exhaust port was misaligned (photo below). I should have spotted it before installing, but I just assumed it would have already been done by the supplier. Anyway, no matter - glad i found it as it explained my exhaust sealing problems.

New piston:
So, i took an old cylinder and used that. It had a sloppy piston, so I put a new one in. Ring gap is text book, and I could put a 0.2 feeler down the side of the piston.

Case leak test was ok:
I made I leak test on the case and it passed. I thought this would have been an issue with the carb tuning, but looks fine.

Exhaust port buggering -1mm higher.
here I decided to do a bit of experimentation. I asked a few questions and read quite a but, but still I'm a complete novice. I thought it might be beneficial to "adjust" the exhaust port to open a bit earlier. I did about 1mm with a dremmel. Of course you will ask how i decided on 1mm. well I pretty much chopped off a chickens head and looked at the blood splatter pattern and that told me 1 mm. it also told me that I should raise the cylinder by about 1mm too. So I decided to follow that.

New Clutch
While the engine was out, I thought buggernut and decided also to but in a BGM clutch. nit a super strong, but something more robust than was already there. I put a new clutch cover too.

Engine fired up - idles well, but crap acceleration.
So engine is back in and started OK. Dies when I try and accelerate or moves off very slowly with no power. also now dies when clutch is pressed. here's what I have checked:

1) compression - 100 psi cold, 120 psi warm
2) plug looks oily
3) changed plug -same low power
4) Old fuel? drained tank
5) fuel supply - flows fine through pipe to carb

So, which of the following could it be:
1) Clutch issue?
2) carb jetting
3) Ive misread the chickens blood and screwed the cylinder. i.e adjustment has lost all my lower end power.
4) Some electrical issue?

If anyone has got this far, I would be happy to hear if you have any ideas on what to try before I remove the cylinder again.

thanks and have a great day

Chief

Makes me think of an air leak between the carb and the engine, did you check if the carb base is flat ? Could be warped.

Also check the gap in between the crank and the pad, using feeler gauge through the admission hole while the carb is off. .10 is ok, .15 average, more is no good.

Some real specialists will surely help you diagnose the problem !

How confident are you that you did the pressure test correctly?

Absolutely sure the piston is the correct way up?

Nothing wrong with the 1mm exhaust or packer. Did you skim the head to get the squish gap back down to stock?

Thanks for chipping in Jack and Colin

I put a new gasket on the carb and used a torque wrench. So confident there.

I'm also confident with the pressure test.

You have put a doubt in my mind about the piston being the right way around. Normally i would be careful of this, but maybe I was in a rush to get it back together. I need to check this.

I skimmed a bit off the head to make up a bit for the 1mm rise of the cylinder, but I didn't measure.

So, I was thinking that I would peep in the sparkplug hole to check on the piston, but if I need to do squish too I should take off the head.

I'm back home.late tonight so I am going to check tomorrow and report back.

Thanks guys

If you take the exhaust completely off and look up the port. You should only be able to see skirt or crown. If you can see the boost hole in the skirt at TDC it's upside down.

So the piston appears to be the correct orientation.

but it's quite wet. so I guess I'm too rich?

Would an electrical issue cause such a thing? I also had the dashboard off and had to reconnect a wire or two. Could it be that incorrect wiring might not affect idling, but could come into effect when pulling on the throttle?

Well the piston is ok, great. If it runs at all, its nothing to do with the wiring.
The oily wetness is a concern. How is the float needle, if you leave the tap on, no puddle on the floor?
When it is running will it rev very high when holding low throttle?
Did you try another CDI?
What is the jetting? Sure the main jet is screwed in tight?

right then,

No fuel on floor with open tap. Fleat needle condition should be find as the carb is quite new

reving is crap. 1/4 throttle nothing, 1/2 to 3/4 starts to react. if I leave throttle on it will wind up eventually.

Jetting: can't remember the idle jet, numbers but dividing the two I got 0 32. I did that as I had a bunch of idle jets and I wanted to put them in an order. if I put in a 0.4, it seems to do a little better, which I don't get as it should be richer. but then why I get the wet piston?

Main stack: I've tried 96 and 100 main, BE3 and BE4 and also 140 and 160 corrector. I have to admit I have not been very systematic as I expected to get back to something more close to what it was before my grand overhaul quite quick.

one thing I noticed was that if I leave the main jet stack out and pull the throttle, it behaves the same as if i have it in. I.e it wants to die when I pull tge throttle.

I didn't check the CDI, does that cause this type of problem? how does that work?

Audio visual aids from today

https://youtube.com/shorts/kAeSOuigkRE?si=6UCdvdW-Z5YdBWnS

Idle jet: 48/140
stack: 96 main, 30 mixer, 140 corrector.

Check the squish. Doesn't sound like it has much compression.
HT lead rubbing on the fan is never a good thing.

Jetting to work from on a 20/20 is
AC140/BE5/100 45/140
Which definitely works for a 125. Yours might need a little richer once it's behaving correctly.

I've put the jetting as you have suggested.

https://youtube.com/shorts/8VDts4gV4KU?si=P0LPZwfVDhnSSy2W

I'm thinking I may need to recheck the timing also. I can't remember what I had before. This time I will write it down.

with regards to squish. if it's no good, I can remove some of the gasket height which would bring it down a bit.

What would you suggest first, check and adjust timing first or squish?

Update.

checked timing it's about 17deg another idle and the moves back to about 20 with some big revs.

checked squish. I have no idea how i missed this, but it was about 3mm. Can't remember it being like that with this cylinder. I thought my head shaving would have brought it down to an acceptable measure. Anyhow, I took it all apart and removed a spacer bringing it down to about 2mm. I'm now going to dig out an old piston which has a bit more height than the one I got in there. hopefully I should then be able to pull away without dying.

Did you say you skimmed the cylinder head?

It's running ok, just doesn't rev right. Stock 125 isn't like a race bike but should rev better than that.

What you know about the exhaust pipe?

pipe is a sito, standard one which is relatively new.

I did skim a bit off the head, let's see tomorrow how it all works. But tricky as can't make noise on a Sunday where I live, but I will do my best to get some results.

Have you run the exhaust on anything else? Or have another to try?

I've got another exhaust but it's for a sprint, seems less robust than the one I have on the PX.

No need to fit it. Just hold it on (on Monday) and see if it revs better.

Roger that, I will putnit all back together tomorrow, then see what happens with the exhaust


thanks for your help mate

Short update,

this morning changed the piston for an older one, which was with the cylinder when I was given it. I've put everything back together.

I did a pressure test and cold I got 150 or there abouts.

This is like 60 more than before and more than I ever achieved with this scoot.

hopefully it will not explode. I suspect that my neighbours are still sleeping, but once I see evidence of movement I will go for the start up and see how things go.

morning, I did a bit on the scoot at the weekend. it certainly has improved and it doesn't die pulling away.

this is what she sounds like on the rev:

https://youtube.com/shorts/jPjBPccMRcg?si=0Q6d-gpjqGZODqB1

I have a feeling it sounded a bit better previously with the newer piston. It seems to sound a bit like something is rubbing, also about half way through the vid you can start to hear ticking sound.

As a test I put in a different stack. 100, be4 and 160. It died going down a hill. I put back the stack that Jack 221 suggested and it's already much better.

any thoughts on finetuning would be welcomed, otherwise thanks to all who have commented.

It should rev quicker, did you check your timing ? It should be 18° if I remember well

Hey Colin,

checked timing. I think it's about 18. depends on the revs. if I remember right, it's around 16/17 at idle. High rev about 20, and looks about 18 for mid range. It moves quite a bit, so not sure if it's really optimal. I'm not looking for racing, just to get around and not struggle too much when I give my kid a lift to school.

I'd say that sounds like a 125 now. They are 6 bhp, so you're not going to get a nose bleed from the acceleration.
Did you check the exhaust is good?

Guessing SI jetting is one of the dark arts. From where it is now. If you up the main jet to like 120. Hold wot in 2nd gear. You'll feel it splutter hard. Step the jet down one size at a time until it stops feeling like it's hit a rev limiter. Then down one size more. This will now be in the fun zone.

Hello again and happy Sunday,

So I made a few adjustments on the p125. I put in a new clutch cable and gear changes are much more direct now with less slop with the clutch.

Now, start up - with choke out, starts first time ticks over, then I jump on. after 50 or so metres, it starts to splutter. I put choke back in and it will conk out when I stop.

WOT - I put in my highest number main jet, which is only a 112. I give it full throttle in 2nd and it hits max revs sounding a bit rattly at about 40kmh. There is no spluttering, it just feels like it reached capacity.

after getting up to speed, if I go to v low throttle, it feels a bit limpy. so not very smooth like very subtle jerking. like it's missing a bit now fuel.

Pulling away when warm- a bit slow to start then it kicks in. so acceleration at low throttle, probably the first 1/4 is slow/delayed. tick over is ok, but engine sounds v weak when the clutch is pulled.

Do you think I need to change my idle jet or just mess with the screw?

Should I get a bigger main jet? feels like going more than 112 on a near standard 125 is a bit ott?

thanks again

Change the AC to 120 and see if it gets rich enough. You didn't tune it so much but 112 is a big main jet for a 20/20. If a. AC120 doesn't flood it there's something wrong.

I will try that in the next couple of days. one more thing I just noticed.

I took my son for a spin sround the block. He is only 10 so not a huge weight. so we are going down a hill and the scooter conks out, just doesn't want to go and will not react to throttle, revs just get slower and slower. it happened going up a hill also, goes ok to start with, but after 50m it starts to slow and eventually we can't go further.

initially I thought it seized, but then we get off and after a couple of kicks it goes again.

As you changed the piston, I would go to original settings with the carb.
MJ 100, BE3, AC 160, 1/2 turn to start with.

Did you check the timing ?
Is the space between the pad and the crank within tolerances ?
Could be a lot of things

96 main is tiny

running problems with a passenger onboard always make me think tank vent is getting plugged or fuel line is getting crimped by the extra weight

Hey thanks,

That thought had crossed my mind. Also, I noticed that after going down hill for a while solo for a while I would have a similar effect, so I started to think that maybe the fuel tube is too long now. it has a slight droop towards the floor after exiting the frame. I will shorten and test soon.

I've nit gone outdoor any more tests of late as weather has been poor. But I will update here once I get some new results.

Hello all and thanks again to all who have taken the time to read and comment.

I thought I would update. So in between the crappy weather I did some tests. I tested with and with the 1mm spacer and also with old piston and a newer piston.

I found that with the spacer, it was difficult to get things running smoothly with all sorts of carb adjustment etc.

without the spacer, it was easier. I think the higher compression is probably helping. So I think I'm at a reasonable good place for the moment.

my exhause appears to keep developing leaks, but when it's tight things feel OK. So I think we are good for now. especially helpful we're the jetting suggestions.

thanks again