Piaggio Xevo 125 fuel system question

[Rick600]

Hi. Please can someone explain the fuel supply system for me. This scooter has the fuel tank under the floor. So I guess there must be some sort of pump to lift the fuel from the tank up to the Carb. Does anyone know if the vacuum operated petcock also acts as a vacuum operated mechanical fuel pump as well as a fuel cutoff valve? There is no electrical fuel pump and no other component that could be a pump along the fuel line. The only other possibility that I can fathom out would be that the engine vacuum itself sucks feul up from the tank, I suppose that is possible too but I can't really see it as the carb is open to atmospheric pressure so it can't really be a vacuum. Can it?

Just for background.....the scooter is struggling to start or run. It is totally stock standard.....I have changed/modified nothing at all. A while ago the starting got harder and harder. I have done valve clearance check and adjustment. (0.1mm inlet and 0.15mm exhaust) I have cleaned the carb. NO jets have been changed, no aftermarket exhaust or air filter. Just as it came from the showroom.

I've checked fuel and vacuum lines for leaks. I also checked the inlet manifold seal. All fine.

Any enlightenment would be greatly appreciated.


Thanks.

[oilyrag]

You could disconnect the fuel inlet line from the carb and put it in a bottle and crank the engine. If it's working you should get fuel in the bottle. You could also attach a different fuel source to the carb and see if it starts off that.

[Rick600]

Thanks Oilyrag. Yes. I already did that. The fuel is pumping through. I am trying to understand the mechanics of the system. How does fuel get from the low down tank to the high up carb. Something ha to move it.....what? There's no electric pump. So what lifts the fuel to the carb.

I appreciate your answer. It is a way to test the system. I'm trying to understand how it works.

Cheers.

[Riejufixing]

[WD Forte]

I played with one on one of my lads scoots
and they do rely on good tight pipe fittings
My theory on how they work:
There are negative and positive pressure pulses in the inlet manifold between the butterfly and chamber
this is the energy source of the pump

In a negative pressure state this works against a spring loaded one way valve to lift fuel helped by ambient atmospheric pressure,
then hold it in the pump as the valve closes and the rebounding spring pushes the fuel out of another one way valve to the carb
Rinse and repeat.
Pressure and flow rate is pretty small but all other things being equal they're usually good enough to supply sufficient fuel to small engines

Good function depends on good pipework and fittings and the valves
are rather delicate so any dirt/rust/contamination in the fuel
will feck em right up
Like any other component they wear, so over time may deliver fuel but not in enough quantity and struggle with high demand situations.
____________________
bikers smell of wee

[Rick600]

Thanks WD Forte. I think you are right. It makes the most sense. I guess that tight vacuum fittings and connections would be pretty critical given that it's operating the pump. One of the connections on the manifold is actually removable. I have some doubts about it even though the system is working. I think I'll try a bit of plumbing tape on it all the same. It shouldn't hurt.

I appreciate you taking the time to write the detailed explanation.

I took the carb apart to clean again. I did that a while ago but obviously I didn't do a very good job as the idle jet was totally blocked and the main jet partly blocked again. I don't know if there's a lot of crud in the tank. I'm going to get a new fuel filter tomorrow and I need more carb cleaner. Hopefully once I clean it all out again I'll get it going again properly.

Cheers
Rick