I bored out the Walbro carburator on a 25 cc. Sears Weedwacker engine to aproximately ll mm venturi (was like a 7 before), of course I took the carb apart and cleaned it thoroughly with compressed air and soap and water, then put it back again on the engine and tried it again, after tinkering with the needle valve screws I got it running just perfect, with a 16x6 Classic Design prop it went from about 6700 rpm to 7200 rpm and with a low idle of about 1800 rpm, good steady idle, smooth acceleration with plenty of power, only thing I noticed differently is that when I'm priming the engine, after fuel gets into the carburator and when turning the prop it makes a sound like if air is compressed and then slowly released, it is hard to describe, like a phhhew! sound, any ideas what is causing this, it is obviously coming out of the carburator when the prop is flipped hard for starting, other than that it runs like a swiss watch.