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.

does it make that noise when slowly turned over?

It is only noticeable when flipped hard over, like for starting and only after fuel has reached the carburator.

If it runs that good, it's probably OK.

If there was anything wrong with the carb, you wouldn't be able to tune it.

Walter,
This sounds like blowby caused by air rushing past the piston. How is the compression?

Dave

Compression is very good, there are no leaks on the crankcase (I made sure every bolt was tight before monting it in the test stand), as I type this, only thing different other than enlarging the venturi size, was that I soldered shut the little hole in the butterfly, maybe this had something to do with it, but I doubt it, I will take it apart again and see if there is something suspicious in the diaphragm. I will let you guys know is I find something, go figure this one! By the way, the noise is not heard if there is no fuel in the carburator.

Where was "the little hole" you soldered shut ???

Walbro's have a little hole on the carburator butterfly that opens and closes as you throttle up or down, I did this in order to have the engine shut off when the throttle is closed, with the little hole open it sometimes tends to keep running (I am not using a kill switch at the moment as the engine is being tested on a stand).

I also think you are just hearing compression leaking by the ring which is normal at "flip over" speeds. The ring might not seal well enough at a "flip over" until it gets some gas/oil to it, so that's why you don't hear it until the engine is primed. The diaphragm is very sensitive to air leaks, etc., and never has enough pressure on it to make this sound so I don't think it's coming from there.

It runs good, so I wouldn't worry about it!

AV8TOR

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.

That's the right hole, I was concerned that perhaps you'd closed off the diaphram hole.