Small Engines (Lawn Mowers, etc.): Sabre white smoke, briggs and stratton, compression check

Question
I've seen a few people post about this sort of problem, and the usual issue is overfilled oil, but am pretty sure that's not my issue.

Here's what our 5 year old 15.5hp 1542HS Sabre (Briggs and Stratton 28N700) is doing.  A few days ago, the riding mower was started, and began billowing white smoke, then it died.  About 10 minutes later, we got it started, but shortly thereafter the same thing occurred.  Then we noted that there was gas in the oil.

I pulled the carbs apart, cleaned them with carb cleaner, replaced a few gaskets, checked the float to make sure it wasn't sticking (no fuel pump but there is a fuel solenoid), and buttoned it back up.  Replaced the oil, spark plug, and fuel filter, and started it up again.  Same problem: runs for about a minute clean, then starts billowing white smoke.  The oil level is now correct.  Did a compression check and got 90# and then 120#, so the compression looks ok.  The rockers appear to operate smoothly.

Suggestions on what to pull apart or look for next?

Thanks in advance.

Answer
We had a similar occurrance last weekend.  Did you remove the spark plug and crank the engine?  I bet there is some oil/gas in the cylinder and muffler.  The thinned oil gets trapped in the muffler baffles...you can so the muffler in carburetor cleaner to help clean the muffler.

Also, check the valve breather.  The thinned oil may be causing the flutter valve in the breather not to function correctly.  This happens often in Kohler engines.

Let me know what you find.
Eric