PX 125 flat spot when in 4th, let throttle go and resume it

[onepieceatatime]

Hi everybody,

I have looked around the internet to try understand what could cause this strange behaviour in my newly restored PX 125 1985 (it has a 2003 all original PX engine, not catalysed): the best way I can describe the anomaly is that when I cruise in 4th gear and release the throttle for a sec, then open it up again coming out of a bend, the vespa is hesitant and bogs down for a second, then the engine starts pulling strong again.
It's very annoying and also not very safe. no problem at all when driving in town at 30mph, but if you're on a little stretch and in 4th around 40/50 mph then you let go of the throttle and open it up again, the one second hesitation always appears.
I am running everyhitng stock and my idle jet is a 45-140 , while the max is 140-be5-96.
Carb is super clean, mix screw is out 2 + 1/2 turns from tight. vespa idles really nice. it smokes a lot when cold, but stops smoking after 5 minutes. in no other occasion the engine bogs down. It always responds promptly to the throttle, except of course in the case I just described.
The only thing I can think of is the length of the fuel line, does it have to be exactly 60cm? or is it ok to give it a little bit of slack? could it be, for example, 10cm longer?
I know that the spark plug should be light brown after a 10 minutes ride, but I happen to be color blind (lol! I even painted my vespa by myself knowing it was red, but not understanding what kind of red exactly i was spraying!), so if you guys know of another system a little bit more scientific than the good ol spark plug color I would appreciate it!
You guys are awesome and here's my previous thread with photos of restoration, if you're interested vespa.proboards.com/thread/6145/vespa-complete-restoration-blog-photos?page=1&scrollTo=48759
Peace out

[henri]

i think a fuel pipes 24" ,but have never measured one .i just connect at carb end an trim so its just long enough to reach tap when tanks lifted .lengthening your fuel pipe is more likely to give problems with low fuel flow .as short as possible is best ,less chance of looping an air locking, worth removing the air hose an making sure its a smooth level run on the pipe from frame to carb box .
ive a similar problem on my t5/polini 152 ,not bogging so much but a hiccup on shutting then opening throttle ,its at 1/4 throttle an is caused by the slide cut-outs .as yours is stock unless youve changed slides or carb it shouldnt be that .
as for a more scientific set up method than plug chops its a dyno run ,H

[pxguru]

I doubt it is anything to do with the fuel pipe. Are the jets screwed in tight? Might be an air leak somewhere.

Post a photo of the spark plug and we will tell you what colour it is

[onepieceatatime]

Hi henri and pxguru and thank you for your prompt replies. I am thinking it might be wrong carburation as I am running a stock 125 cc from 2003 (Min 45-140
Max 140-be5-96) on a non catalytic standard exhaust. I therefore ordered a set of 10 jets (from 100 to 109) a[lus a BE3 atomiser and will soon try to drill holes to air filter and see what happens. I will indeed post the plug photo in here! Cheers guys
PG

[onepieceatatime]

Hey guys,
As promised here's the photo of the plug before any modification so with original be5/96 and no air filter holes. It seems all right to me, which is weird as I was expecting it to be much whiter.

After I took this picture I changed the jet with a 102 and drilled a 5mm and a 7mm hole in the "hheart " of the filter bit I haven't taken a picture. I have to say that the Vespa seems to act exactly like before so problem not solved.

In the meantime be3 atomiser arrived and I'll be trying that too probably on Sunday and take a picture of the plug after a long stretch at fill throttle.

[Gothlife]

Morning.

I have a standard 125 and did this simple mod (drilled out the whole heart) and stuck in a 104MJ and it's running sweet. Probably got an extra 5-8mph out of it and it now comfortable sits at 55-60.

What MJ did you end up settling at?

My question is, do you think it's worth me changing the atomiser to a BE3?

Like I said scoot is running sweet. My only issues are it's sometimes boggy in 1st when pulling away (possibly just my driving) and it struggles to start when warm...

Since drilling out the filter it starts first kick when cold (never did that before) but when it's warm (e.g. when I stop for fuel on my way home) it'll take a good few kicks to start again, sometimes needing to sit for 5 mins before starting.

Could this possibly have something to do with upping the main jet but not changing anything else?

Looking forward to seeing how you get on.

[onepieceatatime]

May 26, 2017 8:02:07 GMT Gothlife said:

Morning.

I have a standard 125 and did this simple mod (drilled out the whole heart) and stuck in a 104MJ and it's running sweet. Probably got an extra 5-8mph out of it and it now comfortable sits at 55-60.

What MJ did you end up settling at?

My question is, do you think it's worth me changing the atomiser to a BE3?

Like I said scoot is running sweet. My only issues are it's sometimes boggy in 1st when pulling away (possibly just my driving) and it struggles to start when warm...

Since drilling out the filter it starts first kick when cold (never did that before) but when it's warm (e.g. when I stop for fuel on my way home) it'll take a good few kicks to start again, sometimes needing to sit for 5 mins before starting.

Could this possibly have something to do with upping the main jet but not changing anything else?

Looking forward to seeing how you get on.

Hi Gothlife first of all congrats on reaching 55-60 mph comfortably, mine is all original 125 (no cat) and it won't pass 50mph at full throttle! How did you do it?

As for be3, I don't think you should put it as I tried it together with a 102 and drilled goes and my Vespa started bogging down at Low revs and wouldn't accelerate fully either. So I revert to be5 and even went down to 100. Runs well now but the initial problem at full speed described at beginning of this post remains 😔

I assume you have a standard dellorto 20/20 in? I would only go to be3 if you upgrade top end with a 177cc I think

[Gothlife]

Morning.

Cheers dude, other than drilling the filter I've done nothing. Think it had a pretty good service just before I got it (about 350 miles ago).

Yer it's the 20/20 in mine. I did have an issue with it bogging down and cutting out due to over fuelling. A mix of leaving my fuel tap open for a couple of days and me not really knowing how to ride correctly and twisting the throttle when sitting etc. Once all the excess fuel was cleared out it ran ok.

I've just come down to a 102 main jet as I was getting a slight bog in 1st when warm and the scooter didn't want to start up again once it had warmed up. It's fine when running but doesn't like to get started.

Think the 102 has helped but it's not perfect and the weather has been abit off so might just be because it's running cooler. It doesn't seem to have effected the top end though, will still hit 60 on flat and ride at 50 all day long.

Not sure if you have tried this (or if it's even a thing to try) but is your throttle cable ok? Might it have possibly stretched and so you are not actually getting full throttle when twisting?

Also, I'm guessing you are mixing the fuel in the tank as it doesn't have an autolube? Is your mix right? (I know nothing about that, without the autolube I'd be in trouble.

Cheers for the advice on the be3, I'll hope off until I at least get an exhaust before thinking about my other jets.

Hope some of that was useful.

Good luck mate.
Kriss.

[pxguru]

Onepiecceatatime don't like the look of your plug. Seems too weak. Grey is the colour that comes after white.

Can you remind me about all the jets on your carb? Atomiser, AC, MJ & Pilot.

Also where your timing is set?

While your at it check your speedo. It should thrash to 30mph in 2nd gear and near to 50mph in 3rd gear, if the speedo is ok. Once sorted it should manage just more than 60mph on a good day.