I've been looking and asking all over. Get different advice. My mower will start fine and run good for a few minutes then all of a sudden it starts sputtering, smoking (greyish) and then die. If I let it sit for a while then it will usually start the same way and after a minute or so do the same thing. If I try to start again immediately problem still there and to get it started I either have to choke or set throttle all the way down to idle - just depends.

I was told it could be a crankcase check valve gone bad and allowing oil back into carb. I don't see a tube or a check valve - maybe it's behind the carb where carb mounts to engine? I took apart bottom of carb and both needle/seat seem OK (replaced last year) and float is still good. Any suggestions?

Try running with the air filter and gas cap removed, just for testing purposes. Let us know if the engine runs better with this stuff off.

Actually, that's how I've been working on it. It makes no difference if I take off the gas cap or air filter.

Grey sounds like fuel, white is oil. Remove the bowl again, after a moment does any fuel continue to drip from the needle?

No. It seals 100% if I remember. I will go out in a little while and check again to make sure but I think it seats well. Also, it's hard to describe the exact color but I think the smoke may be on the grey-white side.