Small Engines (Lawn Mowers, etc.): Lawn Tractor Engine, bridge rectifier, regulator rectifier

Question
I recently acquired a used 14 HP Briggs to put into my tractor.  Got it installed and got it started but gas through out the muffler.  Replaced the solenoid because the needle was all gunked up.  Started but idled roughly (needs some adjustsment on carb i believe).  After I used it for around 45 minutes parked it in the garage.  Next morning I went out and noticed that the little gas that was left in the tank was gone.  Pulled the spark plug and the remaining gas came out.  I noticed that I left the key in the on position and the battery was dead.  Did leaving the key "on" cause the bowl to fill up etc.  Should I change the oil as well?  Thanks

Answer
First change the oil.

Second install a new needle and seat in the carburetor, remove the carb to do this.  Check the float to be sure there is no gas in it.  

You are correct the bowl over flowed into the crankcase, leaving the key on would not do this.

The battery not staying charges could be the key position but also a wiring issue.  When you changed the engine did you change the charging circuit?  Meaning, is there a different alternator/stator on the new engine. How many wires are coming out of the engine?  Is it the same as the old engine?  If there is 2 wires one charges the battery and one runs the accessories. If there is one it charger the battery and the battery powers the accessories.  

The first rule is that all engine stators produce AC voltage.  You need DC voltage to charge the battery and run the electrical system.  This is done with a diode. It is usually in line on the engine side of the plug.  It would feel like a bump in the wire.  

Hooking it up is critical.  Is there a regulator rectifier or a bridge rectifier in the circuit?  All this information is in the electrical schematic in your owners manual.  If you don't have it contact the tractor manufacturer to get it.  You will need the model and serial numbers of the tractor.  

One thing to check if all s hooked up correctly is your grounds.  There are multiple grounds in the electrical system.  You could use an amp meter to measure the current to ground.  With the engine off place the meter between and the frame of the tractor.  With the key off there should be very little to no current moving to ground. If there is it is another ground.

Good Luck