hi guys,
what could've caused the nitro in the tubing to 'back flow' when i crank it on the starterbox?

it's was fine 10mins ago but after it stalls and when i tried restarting, it just wouldnt start anymore and i notice the nitro would 'back flow' and my nitro line will be empty of nitro...

even if i tried covering the exhaust and managed to pump nitro into the carb, the nitro would 'back flow' again when i crank it on the box!