YZ250 popping noise

[madmanmechanic]

Hello to all....i just bought a 2000 YZ250 from a private party. i ran it about 4 hrs the first night off and on and it ran AWESOME. the problem i am having now is mid-range...it is making a "popping" noise. the idle is fine and so is wide open...but cruizing around with light to medium throttle, it hesitates some and pops. not out the exhaust or air filter...sounds weird. i changed the plug, cleaned out the carb, the water is up and has never gone down, i mix 40.1, changed the fmf to the spark arrestor, pulled the head off, and saw a very small what looks like a crack on the passenger side..way down below but am not even sure if it is a crack. one other thing i noticed is the rubber hose from the crankcase right below the valve, is smoking just a bit...and sometimes has drips...im really thinking that this "crack" is whats shoving a little water in the cylinder. does anyone have ANY suggestions??? im getting to the point that my good deal...is a lemon.

thanks--corey--

 

[nickyd]

or your jetting is off -
pop=lean. move clip on needle down one or bump a size up on the main jet.

the hose below the valve? what valve? the powervalve? that's a vent line. suppose to be there and possibly smoke