Today I took the carburator apart and replaced the diaphragm for a new one, everyting else looked OK, bolted it to the test stand and the noise went away, go figure! Top RPM with a 16x6 is 7200 rpm and a low idle of about 1600 to 1800 rpm, not bad for a 25 cc, the opening of the venturi really made the engine come alive as far as rpm goes, it was only doing 6800 before. I'm happy!
Thanks to all for the replies, you guys are great to correspond with.