4 stroking at 0 throttle-1/4 throttle

[GearNut]

The gas tank has to be vented.
As the gas exits the tank air must be allowed to enter the tank to displace the volume of fuel that has exited the tank. Without a vent the tank will only develop a vacuum inside of it and the gas will no longer flow out.
This is the same principal as holding your finger over a straw to hold a beverage inside of it.
The beverage stays inside the straw until you vent it.

 

[Goat Herder]

Gearnut's senerio plays in with pressure building up in a tank on a hot summer day and venting into the carb as well too.

 

[mapbike]

Gotcha, except if stock tanks are not vented, why does mine have to be?

Remember you're dealing with chinese products here..... not doing the little things is how they're able to sell stuff cheap, they rarely send things over here that have all the I's dotted and the T's crossed.

Map

 

[mapbike]

The gas tank has to be vented.
As the gas exits the tank air must be allowed to enter the tank to displace the volume of fuel that has exited the tank. Without a vent the tank will only develop a vacuum inside of it and the gas will no longer flow out.
This is the same principal as holding your finger over a straw to hold a beverage inside of it.
The beverage stays inside the straw until you vent it.

Excellent explanation GearNut...

Map