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Lets start with the easy things first as this may be a simple fix. If it does not work we will dig deeper until we find the problem.
First I want you to remove the gas cap and look at the underside of it. Is there a metal disc on the inside of it? Next please look inside the gas tank with a flashlight and check to see if there is anything inside the tank (there should not be anything)
Now we can test this by just running it outside. Let it run full speed and just watch it. I want you leave the gas cap very loose.
Let me know what you find and we will continue