How did gas get in the oil in a Briggs stratton 16.5 HP engine?
It is normal for a little gas to be present in the oil. This is
why it is important to change the oil at least once a season. If
there is a lot of gas in it then I would say that the carburetor is
malfunction or isn't properly adjusted. Too much gas flow and it
leak into the oil.
On my riding mower, the problem was originally caused by water
in the fuel. This can originate from your local gas station, or in
my case a gas cap with a "fuel gauge" built into it, which
eventually allowed water to leak into the tank.
The water settles to the bottom of the carburetor float bowl,
eventually rusting the float needle, which no longer shuts the fuel
off completely. This allows fuel to drain into the crankcase and
contaminate the oil.
My solution: Drain and replace oil. Clean float needle until it
successfully shuts fuel flow off (or replace needle if necessary).
If you have one of the aforementioned "gas gauge" type fuel caps,
toss it and replace with a standard vented cap. And MOST
IMPORTANTLY - if your mower does not have a manual fuel shutoff
valve, install one ASAP. They are available at any mower shop for a
couple bucks. Simple inline installation, just cut gas line in a
convenient location and plug the valve in place. Now you can shut
the fuel supply off when not in use.
Of course, I figured all this out AFTER I replaced the
connecting rod and patched the hole in the block!