Just a couple of thoughts. Starting up and running for a minute almost sounds like it's starving for fuel. (especially with an "indication of a fire" at the intake manifold. Sounds like it's a really lean mixture with an intake gasket leaking. Check the gasket and manifold for leaks. Might also check the float level too. Any help if you pull out the choke? Also....when was the last time you checked the fuel tank for water? If it sets outside, or if it's been really low on gas, condensation may have built up in the tank enough to have water in the tank, or if it's "old gas" that may also be a problem.
And I know you said you checked the filter, but have you checked to see that there is no blockage in the line from either the tank to the sediment bowl or from the bowl to the carb?
Hope this give you some ideas. Will see what other's have to say.
Dave of southwest michigan