Modern Vespa

I replaced my old bad petcock on my et2, but now the bike struggles to run. If I let it sit for about 10 minutes it starts and I can drive it around but if I let it idle it dies and doesn't run again for another 10 minutes. All I replaced was the petcock, and put brand new fuel and vacuum lines in the carb. Before this it ran just fine. What is going on???

What was wrong with your old petcock?
Your new petcock may be defective, or the intake manifold is not producing enough suction to engage the fuel valve on the new petcock. Try sucking on the vacuum hose to see if you can manually trip the valve.

It would leak fuel without the engine on and I thought it was the reason that I lost power at full throttle but it turned out to not be the reason. How likely is it that it came defective?

test the new petcock as i described above, that's the only way to know it works

Look for a kinked fuel line or breaks in the vacuum lines.

I replaced both of them with new versions of the lines it was using when it was working

Trace your fuel line, and be sure it did not kink. I had a similar problem with my GT200. New fuel and vacuum lines, it would run for 3 blocks and die, I would push it back home, and 5 minutes later it would repeat. Ended up being a sharp turn in the fuel line from the petcock to the pump. It kinked up behind where I could see it. Sitting, fuel would seep in and fill the bowl. After it ran for a short while, fuel flow couldn't keep up and it would die.

I tested it and it was fine, fuel flowed nicely while I was sucking. And here's a picture of the lines so you can see there is no visible kinks. Really confused on this one.

it is possible you have a vapor lock. Try it with the fuel cap off the tank.

Thank you for the photo, yep no chance of a kinked fuel line. I suspect your problem lies in the carburetor at this point. Let's rule out the petcock. Pull the fuel line at the carb and shove a 6mm bolt into it to block it. Rig a temporary fuel source out of a piece of hose and a syringe or small squeeze bottle. Attach to the carburetor, fill with some gas, add a few drops of 2t oil and see what happens. At any point have you sprayed around the carb and manifold with ether while running to check for vacuum leaks?

Unfortunately this made no difference. And no I have not sprayed around the carb and manifold because they are all brand new parts. The carb is new, the petcock, all the hoses, the manifold, the rubber for the air-filter, the airfilter itself, and the spark plug. All around 3 months old. Should I spary it anyway?

I also found this video of a tao tao scooter that seems to act the same as my vespa, struggles to start (mine struggles unless it sits for about 10 minutes), and dies at idle but runs fine at higher rpm. Could a valve adjustment be what I need? ?si=laPmS0QGxGEQbLEm

Unfortunately ET2 doesn´t have any valves to adjust, as it has a 2-stroke engine.

Another (possibly) unrelated issue is you are missing the rear mudguard. Your fuel line shouldn't be rubbing on the rear tyre like that.

See if it fires with ether. If not, check for spark. If there's spark, change spark plugs and try again.
If not, fix that.
Next check the compression. Most of the ET2s have been sitting for a while and the rings corrode into the cylinder wall. Over a surprisingly short time,this will lower the compression to the point the bike won't run.

I checked the spark plug and it looks like it's pretty black. Could this be causing the problem or is this average for a 3 month old plug.

Changed it and there is not difference

That's a Denso. They're a very good plug. The temperature of the plug has to get above 450 degrees for the plug to clean itself, so if you're not running it until it gets fully warm, you are going to have issues with fouled plugs.

Think I found out why it won't idle 🤦‍♂️🤦‍♂️