I've had engines go suddenly bad, it was dirt in the carb. Try pumping fuel through the carb at idle and WOT to flush it out. It helped me.

I have a Magnum 52 4-stroke (same as ASP) and I just about gave up on it. I have almost 2 gallons of fuel through it and I have concluded that it is an easy starter, but idle stays too high unless I lean out the LS needle and reduce WOT power. In the end, it will be a basic low power engine. Easy to start, but weak.

The money I saved in the price of the engine, I spent on the extra fuel to break it in (+aggravation). Just run it on the bench with old fuel to break it in (2-gallons from the concensus here).