Modern Vespa

Hi,

New to Vespa, just picked up a PK125XL elestart a week ago and it's been super fun to bop about on but I have a few issues.

One of those is that after letting of the gas at high revs it hangs and won't go come down unless I break the engine speed with clutch+breaks. It will rev up super high or just give me undesired cruise control at random times. It's not 100% of the time but it is most of the time.

I suspect a vacuum leak as I can't get the throttle to stick up no matter what I try.

Checking the plug it seems somewhere on the edge of too lean, it is a perfect brown-bronze on one side and slightly pitted and black on the other.

Pulled the carb and checked the jets, what I belive is the main reads 80, the other one in the bowl reads 60(is this the choke jet?) and the one that screws in from the side reads 55. These seem suuuuper rich compared to what the normal PK125 has from what I found online, but I could not find anything on the XL.

I suspect the intake manifold gasket is the culprit and the rich jets feed the issue further but I'd like to hear from someone who has worked more with these engines before I start the whole procedure to get the intake manifold off

But the XL125 is fitted with SHBC-20L carb. The jetting that carb comes with as standard from factory is 76 Main jet 50 Idle jet 60 Starter jet; this is factory correct for the 75cc XL, piaggio book speed is 80kmh for the 75cc and 90kmh for the 125xl. It sounds to me your jetting is correct as factory standard for your scooter. They've (piaggio/motovespa) upped the pilot by 5 pts and main by 4 to compensate for the extra cc's.

For reference https://www.dellorto.co.uk/shop/dellorto-motorcycle-carburettors-parts/complete-carburettors-dellorto-motorcycle-carburettors-parts/shbc-18-to-20mm/r0909-shbc-20l-piaggio-vespa-p125-ets/

Ah well then, that was news to me

I haven't found much good info on the PK125XL so I went with what the previous owner told me mostly. According to him the carb is a SHBC19.19, and that's what SIP Scooter shop seems to recommend too.

Good to know!

After cleaning the carb and replacing the gasket at the top the vacuum leak seems to have possibly improved slightly, doesn't seem to stick at high revs as often now but definitely still does it.

Pretty much convinced that the leak isn't to do with the carburettor now at least. There is a pretty significant amount of oil around where the intake manifold mates with the case so I am suspecting a torn or missing gasket there.

Also looking around there seems to be an "intake manifold bushing" at least on the other PKs, where would that go? I can't see a corresponding part on my bike. It's just the carb that sits on the manifold which goes through the rubber boot and mates with the crank case.

The carb should have an embossed identifier on it , eg 20L or 19.19 . The XL 125 has pretty big ports on it so would imagine the reason for bigger carb , the ETS has a 20L the the XL followed the ETS . What's the state of the petrol tank interior ? Does the carb have any movement by hand when you sit on the tank ?

I'll check and report back with what the markings on the carb say. As for the fuel tank it is pristine inside, no rust or anything.

I'll check if the carb moved when I sit on the tank, I would assume it will as it is attached to the engine and not the frame

Im trying to establish whether the carb clamp is binding the carb to the inlet correctly. They work loose and there lies an air leak area. By sitting on the tank the inlet will push out and its easier to see if there is any carb movement, its also easier to tighten up the clamp.

Carb is SHBC 20L! Not 19.19.

Cant move it by hand, even if I sit on the tank. Maybe a tiiiny bit of movement if I really manhandle it. I've had the carb off to clean it and take the pictures in the OP. Made sure that it's properly tight and as far onto the intake manifold as I could push it when I put it back on.

Hmm maybe it is the jetting but it's going through 4.4L/100km of fuel, that seems excessive. Before I bought it I was under the impression that it would only sip 2.4-3.8L/100km.

Also haven't found out anything new about why it hangs at revs, still suspecting the intake manifold gasket.

Got parts yesterday, put in a bunch of hours of work today and I think I figured it out!

There was 1.75 intake gaskets

There was the one they put on when they put it together to put it in the frame and there was most of the one that was there when the engine was removed

Took a lot of work to get the 0.75 extra gaskets off but I got it, replaced it with a brand new gasket and I didn't experience any issues on the ride home even when trying my best to make it stick! Let's hope it stays good!

Think I picked up a bit of midrange power too, feels more peppy!