Can you ride a bike 1200 1994 sportster after leaving th egas on all night or do you have to change the oil?

Leaving the fuel petcock on should not pose a problem unless the carburetor float valve is bad. Chech your oil level is it higher than the last time that you checked it? does it smell like fuel if not then its ok, is so then its best that you change the oil and get the float valve repaired. Remember that all small engines need their oil checked every day before startup.

ANSWER: leaving the gas petcock on all night will have no effect on your oil what so ever since all Harley have what is called a dry sump oil system. That is, rather then storing the oil in the crank case, the oil is stored in a containor separate from the engine. If you have a weak float valve, the result will be that all your gas will leak out through the carbs over flow vent. If you came out in the morning and there was not a puddle of gas under your bike, then you are just fine to ride it normally. Don't make leaving the petcock open all night a habit though.