[WSUcommuter]

I take it that your power washer is not new?

If this is the case, many Briggs & Stratton engines have diaphragms on the carburetors that tend to get weak and start losing seal. This often results in engine loping when extra air gets sucked into the carb. Also, you stated you cleaned the carb. Did you try running compressed air through the needles? Often they get clogged and a simple spray of carb cleaner is insufficient. Hope this helps!