Our Mantis 2-cycle tiller is one of our favorite tools. Runs great although my wife has difficulty starting it when its cold but does fine once its been run a bit. We do use ethanol-free gasoline and run it dry before storage.
Sounds like the same machine and age that I have. I recently had to replace the primer bulb. The old one was cracked and leaking air. My first thought on yours was a dirty, partially plugged, fuel filter in the tank. But now it does sound more like a gas cap that isn't venting as it should. Good luck with it.
The fuel filter felt is the problem. Replacing your fuel filter WILL NOT permanently cure the problem. The felt fibers swell and will not pass enough fuel. Turns out, these Mantis fuel filters already contain a fine metal screen filter. It's under the felt.
I do the exact same thing now. Was having issues with it running. Bought a carb off of Amazon & it fired right up & ran perfect. $14 on Amazon & the kit comes with 2 extra bulbs, a plug, new fuel & breather lines. Can't go wrongFor the little I run my Mantis, I just put canned Husqvarna pre mix fuel in it.
That way I don't have to worry about it and it ALWAYS runs when I want it to.