The power valve is probably blown out due to a backfire throught the carb at one time. It doesn't take much, a hesistation when coming off idle when cold & poof! Cutting the choke horn off (assuming it was done correctly & not butchered up) shouldn't cause the power valve to open early. You also want to make sure the idle circuit isn't blocked otherwise that will cause the fuel to pull over.
I'd put a new power valve in it & clean out the idle circuits real good & go from there.
