Modern Vespa

The centerstand springs are covered in Polini's instructions. Just remove the clips from the crossmember and hook the springs directly into the holes where the clips used to go. Worked fine for me.

Hi Guys
Firstly thank you for all your input,veryy much appreciated. Finally solved the problem, it turned out to be a very slightly damaged thread at the tip of the bolt which engaged when dry fitting off the scoot but on the scoot with a fractional mismatch refused to thread. New bolt solved the problem.
Next question Jet stack on my px is the standard 140/BE5/96 main. Ive changed the jet to a 106 ( 112 and 110 just flooded the engine) and seem to have it running ok to my ear (but I doo wear hearing aids so not sure how good that is) Is there any procedure/guide That is fool proof or does it come with experience?

Cheers Andy

Last edited by andyx on Fri Jun 24, 2022 8:30 am; edited 1 time in total

What exhaust are you replacing? Jets on a SI are so easy to replace I'd start with that 112, make sure it's running rich. Should be to big, guessing you won't get around block as soon as you open throttle past 1/2 it'll just bog down rich. Leave motor cowl off, pull over replace jet with a smaller one and try again. Depending on sizes you have start working your way down by a size or two. I know you can get singles 112,111,110 etc in that small size, not sure what you have. It's only n time and a good learning experience.

Thanks Chris as you rightly surmise 112 was way too rich. I'm at 106 now and it seems to be running wellBut not keen on a lock up, any foolproof way to be sure I've got the right jetting or is it a case of feeling my way aroound to a degree.

Nothing is fool proof , I'm a technician by trade, to measure is to know, is my philosophy.

1) CHT - cylinder head temperature - it does work especially for longer sustained runs, not as fast as EGT. Cylinder head mass has a larger mass and slower thermal change. That has left me with a seized cylinder previously. Gauge was still climbing past 350°F as cylinder locked up. I like it better for a stable or sustained reading.

2) EGT - exhaust gas temperature - quicker reaction then CHT as it's measuring exhaust gas and only has to change thermocouple mass in exhaust.

3) Wideband AFR - air fuel ratio - quicker then my EGT - it's a love hate relationship for some. I have been jetted so lean it just showed said lean and I stopped and turned it off immediately. Cylinder head and EGT hadn't had a chance to read anything.

4) Plug chops and cutting plugs open, can be a pain the rear but do work.

Thanks Chris, got a cht probe, which I'll fit when I replace the rear shock (next job).
Just want to avoid a seize and chucking my 66 yearold body down the road

Yeah, wouldn't be any fun at 49 either.

How did you arrive at 106? If you go up one jet size how does it run? I think you said your missing a 108? Ok I see you edited previous post, you went from a 110 to a 106. Theoretically you should be ok, temperature readings would be best, maybe try a 108 if you can get one, best you can do is run on rich side for safety.

I've got a 108 so I'll try that over the weekend but what will tell me if a jet is too rich?

Without indicators such as EGT and CHT, as long as you can still rev out to max rpm, cooler you will run. As you get leaner you can still rev to maximum rpm but you'll be hotter, to hot everything expands, to hot aluminum melts.To hot, oil reaches it flash point and no longer lubricates cylinder walls.

How on earth are you considering that jet size? That's way too high. A road 2.0 sip will be no more than 102 on your jet stack and that's with derestricted air filter too. The polini isn't as powerful top speed as a road 2.0, it's a good 5 mph slower and does not rev on like a road 2.,Without seeming to be arsey we are repeating the info you ask for when it's already been given . Anything like a 108 will rich bog like hell, your plug will be black and wet. It's a standard 125 we are talking here with a rather mediocre box pipe on a standard cylinder . 101 or 102 jet !!! You are going 12 pts over standard jet with a 108, even your polini instructions will state 4 pts over standard !

Hi ferriswolf, thank you for your input but the 106 isnt bogging (had to google that) with the polini and the drilled filter and I'm real new to this and want to avoid seizing the motor and me sliding down the road on my arse.
I'll try the 108 and if it bogs I will have an idea where I am.
Until I get the cht fitted I will err on the side of rich.
Will let you know where I end up

I agree with ferriswolf. If your only mods are a Polini box and a drilled airfilter, your main jet should end up in the 100-102 range.

When you are dialing in a main jet, you are looking for splutter (better descriptor than "bog" in this context I think) at max rpm, i.e., wide open throttle in 3rd gear. It took me a while to learn the difference between a motor that was spluttering and a motor that was revving all the way out, and it is hard to put into words. Of course there is the actual splutter. Beyond that, the best way I can describe it is splutter will feel like the motor is hitting a wall, revving out will feel more like the motor has reached the end of its tether.

Yes it's a gurgle sound , not smooth . Like hearing somebody gargling . You should gauge the plug colour and not going higher on the jets till you splutter or lose speed as not enough air and too much fuel. By all means go to a 104 but you will still be Rich. Then work down.

Thanks guys for your advice and your patience, next step I think is to fit the cht as a back up while I learn to work out what the engine is telling me.
As
before I will keep you updated with my progress on this learning curve.

You have px 125 2003 , I have px 125 2003, it will be an MY disc model .
When I had it standard other than a sip road 2.0 and a derestricted air filter it's jetting was 101 and perfect. It's as simple as replacing your main jet and sticking on your air filter. This is a five minute job. Then you whack your exhaust on, about a 45 minute job. You really don't need a temperature reader. Your scooter will do the same top speed as it did before however with the polini box it will be more eager and torquey in third and from third to forth where it will reach its maximum hp as it can't rev on . If you had a road 2.0 pipe , jet would be 101/102 depending on climate , season as it does have a better top end over standard . I've run both pipes matey preferring the road 2 . I now have a dr177 bolt on and this is where the experiments with jetting , air correctors and atomisers come into play and where a temperature meter could be useful however I have learned with help on this forum how to plug read across all running speeds and the folk on here gave me the likely base jetting info for my kit as they have either got that kit or used to. So yes learn at your own pace but accept the giveaway information to start you off with and keeps it simple and cost effective .

Thank you, I am taking your advice, tomorrow I will be removing the tank to replace the rear shock, while I'm at it I will fit the cht gauge and replace the throttle cable and start coming down with the jets. For me it will be interesting to see the correllation between temps and jet size and performance. Even with the wrong jet in the engine has a lot more botom end and I hope/expect it will improve as I get close to the optimum setting.
It seems to me from all the reading I have done on this forum that developing a 'feel' for the engine is invaluable and thats what I'm trying to to in my cackhanded way.
Please dont take offense if I seem to be a little slow following your advice I am just tring to do it in a measured way that makes sense to me.
Please stick with me as I know your advice/help it the future will help me when I fit a kit, which will be neccessary if I am ever to make the sidecar outfit viable.
I see we live in the same county, maybe we could meet up for a chinwag( coffee's are on me)

You fancy one of these ?
https://beedspeed.com/products/vespa-cozy-side-car-zeppelin-black

Nice, but they also do a 'rocket' shaped one which I'm tending to favour.

A question that has just occurred, My engine was rebuilt 600miles ago(rebored cylinder etc.)
Should I still be running it in and if so whats the best way?

600 miles is plenty for running in.

Thanks for that

Ferriswolf
Think I have found an issue which is possibly affecting the jetting?
Removed the carb and found the gasket was mis fitted, cutting off 20% of the intake.
Any thoughts as to how it would affect the fueling?