I replaced the leaking carburetor on my kx125 2 stroke with a carb I got on Amazon. Brand name is Koso, it's Chinese. It worked great for about an hour on the track then the bike just died and wouldn't start again. I took apart the carburetor and I noticed there is only about a teaspoon worth of fuel in the float bowl. Not enough to even reach any of the Jets. Next, I took out the float and the pin to the fuel line, thinking that blocking fuel flow. With this setup I tried starting the bike opening the fuel switch little by little. Not starting again. I checked the float bowl, again no fuel in it. I blew some air through the fuel line and it wasn't clogged. But it seems that it requires vacuum to pull fuel out of the fuel line. Is this correct? I was assuming the carburetor bowl should fill up by gravity until the float closes the fuel supply. Or does it require vacuum to suck fuel out of the fuel line?

Things I've already tried: 1. Check that spark plug gets spark 2. Adjusted all the screws on the carburetor 3. Connected the fuel line from bottom of float bowl to the overflow opening, and another one from top of bowl to the opening on top of intake side. This is how it came in mail, and I didn't mess with these since the bike was running for a couple hours.

It seems pretty obvious that the issue is no fuel getting to the Jets. All in all I took the carb off the bike about 30 times. Not sure what else to do. I can take a drill to the opening for fuel and open it up a bit so it goes into the bowl by gravity. But I feel like the problem may be a vacuum leak somewhere. All the gaskets are in place and visually I don't see what could be missing.

Any input is appreciated. Thank you

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    I don't know the precise design of the bike's fuel system you are using, but a gravity-fed carb can have trouble if the tank isn't vented. Sometimes, cracking the gas tank cap can alleviate the problem temporarily (as a test). And, no, not in my experience is there supposed to be a vacuum. – mike65535 Jan 1 at 23:55

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