The carb on the popular Harley-Davidson Sportster can be tricky, causing an overflow of fuel.
Remember to turn the gas off on your Sportster, even when just stopping for a few minutes. Unlike a car where the gas must be pumped into the carburetor, a Harley has a gravity feed where the gas tank is always above the engine. Many new Harley riders might not be aware of the potentially expensive lessons that older riders know can happen when you are not careful.
When the petcock is left on, the float in the carburetor is the sole mechanism to prevent cylinder washdown. Something you really don’t want to experience. Gasoline, while a derivative of oil, is a solvent that dilutes and incapacitates oil’s ability to lubricate. The float on most modern new carburetors is pretty reliable and it’s function is to keep a certain amount of gas ready for use in the carburetor at any given time.
The float is normally located in the float bowl on the bottom of the carb. If a piece of grit locates between the float needle and seat (believe me, it happens) it will change the height of the float and the amount of gas in the bowl. The excess gas has two places to go.
One is out of the overflow vent or tube onto the outside motor. While this can increase the chance of a fire, it does not hurt the internal motor components. The other route is down the carb venturi, through the manifold and intake ports and into the cylinders, where it very efficiently cleanses the slippery oil off the barrel walls, pistons and rings. As the gas continues along, it leaves exposed metal surfaces with no protective lubricant surface.
Its gravity-fed journey eventually will take it to the bottom end motor cases, where there are only about six ounces of oil at any given time. This oil is there to ensure that all bearings and bushings are properly lubed. Gas fills the cavity and nullifies any effect the oil will have when the bike is fired up.
If it happens to this extent, the gas/oil, will be spit out onto the ground from earlier Harleys. On newer Evos, it will go out the vent in the carb breather cover and on to the motor as the gas almost goes full circle when the bike is first started.
At this point, the oil must be changed, (as some of the contaminating gas will also be pumped back into the oil tank), and the engine flushed of gasoline. This is good in that a problem is detectable and therefore can be dealt with.
However, if only a little gas seeps through each time the gas petcock is left on, an insidious washdown can happen over time without your realizing it. It will cause you lots of inconvenience and money as the motor will require a total rebuild.
At least once a year and every time the gas line is disconnected for any reason, the petcock should be observed to see if it is defective or worn out. If possible, fill your gas tanks, turn off the petcock and leave a pan underneath it overnight to see if there is any seeping. A weak float combined with a worn petcock has caused the destruction of many an older Harley engine when the symptoms were there.