Modern Vespa

had a problem where while cruising along the engine would quit running (not getting fuel). after several minutes i could get it running again and then shortly down the road it would stop running again. disconnected the fuel line from the tap and attached an extension to the fuel line and confirmed that fuel was being delivered when fuel valve was turned to "on" and "reserve" and stopped when closed. removed carburetor and disassembled it and confirmed that all jets and passage ways were clear. reinstalled carb and when i turned on fuel to see if engine would run notice that fuel was running down the back side of the engine onto the ground. impossible to see under the airbox but it appears that the fuel was exiting the airbox through the breather port in the float chamber. disassembled the carb again and investigated the float and needle. the needle had many years of use so i ordered a new one and replaced the filter gasket as it was not sealing properly preventing me from holding a vacuum through the fuel tap. i also rigged up the float and fuel tap to run water through to test the float and needle function. the water flow stopped when the float was raised sufficiently (well below the top of the float chamber). reinstalled the carb again and no leaking of fuel and engine ran normal. went for a long ride around the neighborhood with no problems. several days later went to go for a ride and the original problem resurfaced (run then quit running wait several minutes then runs for a short distance and quits). what the ****!! any ideas...desperate.

At the risk of offering useless advice…

Experienced something similar once, turned out to be this screw vibrating loose and puking fuel which ran out the back of the air box.

Vent hole in cap plugged ?

Fuel hose must be no longer than 55 cm.

Carb tightened down correctly to air box, carb gasket ok, do you have a warped carb airbox, carb fuel inlet screw ( as in the previous picture) bolted down, small gasket to this screw, is it missing? Incorrect fuel line length?

This^
If your fuel tank cap is clogged, it will not allow air into the tank.
If the tank can't get air, it acts like a straw with your finger over the end - and won't drain fuel into the carb.

I like it. also, fuel hose not being routed properly.

Typically, you will run out of fuel and after a brief pause, the carb will fill up again and repeat. The fuller the tank, the sooner it stops flowing. Or run it with the cap loose and see if it doesn't quit.

excellent suggestion Moto64 and charlieman 22, unfortunately cap breather holes must be open as it made no difference with the tank cap open.

what's the logic behind fuel line being no longer than 55 cm and how its routed?

i've had this same fuel line for two years without any problems.

carb torqued down to 10 ft lbs.

i think i'm going to run the engine till it stops again and then pull the float bowl
cover and see what going on in there.

As the fuel is gravity fed, if the fuel line is too long or not routed correctly you get fuel starvation when your tank is low on petrol. The scoot will run OK with a full tank, but as you get under half a tank, if you wide open throttle and get up to around 50mph it will suddenly starve and splutter then kick back in again as the speed lowers. This is quite nerve wracking if you are overtaking on a dual carriageway and you suddenly lose power with a car driver tearing behind your arse, experienced myself and my line was three inches too long after Id fitted new one and not quite got it right.

Maybe your fuel tap is defect?
You could try replacing it as they are relitively cheap.
I also had issues with fuel line length problems, so as mentioned make sure the length is ok..

Catching up, all the above suggestions are solid things to check.

A bad/worn float needle would not produce the behavior you're describing. It would cause flooding if you parked the bike with the fuel tap left on, and possibly cause bogging to the point of being unable to get past 1/4 throttle due to excess richness until the motor could get going fast enough to consume the fuel flow.

I assume that at this point, the filter screen in the carb is clean, so we can rule that out.

Likewise, you said that you validated that you have adequate fuel flow through the line when disconnected. That's good.

Can you confirm that the fuel line's routing is such that it's a "U," i.e. that there are no humps where an air bubble could form? That'll produce the behavior you're describing, but won't account for the fuel leak.

Now all that being said...have you confirmed that you have spark when this happens?

I forget who first said it, but, "90% of all carb problems are actually electrical."

You could have a bad CDI, too.

On my px There's a brass coloured metal u bend bracket under tank and the line needs to thread
/rest through there. I also replaced the tap with a sip 2.0 as this also helps gravity issues .

To be precise the OP checked fuel flow from tap and not from the hose.
And flow must be checked on the carb level without any extensions. I had problem with my 50 Special. On speed above 60-70 km/h it start to ride unevenly, chaotic jerks and stall sometimes.
I spent three evenings playing with the carb, jets, intake manifold, cylinder head, ignition, etc.
Then I changed HT wire and problem was sorted out