Hi Barro, welcome to Outdoorking.
That is an unusually short cycle time for hunting, but I think it likely that is nevertheless what it is. Please clarify one point, though. Does the cycling cease if you apply the choke slightly? If so, the problem is likely to be lean mixture. In that case if the carburetor has not been disturbed recently - in particular by loosening the attaching studs - the most likely problem is dirt or gum in the carburetor. A dirty fuel filter is also a possibililty, but a somewhat less likely one, because as I understand what you have said and demonstrated, it cycles immediately after it has been started, whereas an upstream fuel flow restriction would only cause lean mixture after a delay while the float level in the carburetor dropped below its normal level. On the other hand, if the carburetor mounting has been disturbed, leaking insulator gaskets would be a strong possibility.
If you cannot stop the cycling by applying just the right amount of choke, it is unlikely to be lean mixture and we will have to go back to first principles. The solutions in that case will probably be found in the governor mechanism inside the crankcase - but that is relatively unlikely at this stage.
