It's perfectly normal to smell exhaust fumes and other petroleum-based odors around a dirt bike even when the engine is off.
But if you smell straight-from-the-pump gas while riding stop subjecting yourself to toxic fumes, get off the bike and figure out what's wrong.
Because you've got a problem.
Certainly the smell of gas is perfectly reasonable, you might say, considering you're sitting on top of a tank full of the stuff. Yes, to a degree that's correct but a gas smell that fails to dissipate, indicates a leak somewhere. Don't worry it's unlikely to blow but leaking gas around a super-hot engine should give you some pause. It's best to find the leak and make the necessary repairs. Besides you'll save on gas money, too.
A number of causes exist for leaking gas from the obvious to ones that require more detective work. A small leak might take some time to locate so if you're having trouble, grab some patience and hopefully we'll help point you in the right direction.
Did you spill when filling the tank? This is the easiest solution because you should know right away that you overfilled or spilled gas. Fumes stick around a while even after fuel dries. Don't think toughing it out equals manhood. Sure it smells kind of good (lots of people like the smell of gas) but it's fatal in high long-term doses but short-term exposure causes headaches and other health issues. Wash your fuel tank and any other areas the gas touched on your dirt bike to rid yourself of the noxious fumes.
Smell gas? Check the fuel vent cap.
Next check the gas cap. If the cap is damaged or worn out, fuel leaks easily leaks through any porous opening simply from the jostling of riding. Small amounts likely evaporate quickly so you might never see actual liquid oozing from the fuel tank opening. Check the gas cap and the gas cap gasket - if either appears to have had better days, replace it. Also, check the fuel vent cap. Is it clogged?
Now look at the fuel line. With the motor running, roll the throttle a bit and check for leaks between the fuel tank and intake. The clamps and hoses tend to deteriorate, turn brittle and leak. While there, you might as well check the fuel tank, too. Old age and crashes tend to wear out seals and cause punctures.
If your dirt bike runs a petcock valve, check that for leaks.
For the 2-stroke crowd, a leaking carburetor is quite common but determining the cause sometimes proves tricky. Did you recently tip the bike over too far? The bowl overflow hoses might drip. How about a recent carb tune-up? If you did it wrong, like an incorrect float height, you'll cause a leak. A bad float bowl gasket also leaks gas.
Lastly, if you have a fuel-injected bike (mostly 4-strokes) with a throttle body, check out the fuel pump. It's inside the fuel tank. Like everything else dealing with fluids, it has a gasket and gaskets leak. The gas lines and connectors for the pump also wear out and leak. Carb and Throttle body intake boots or manifolds also break or the accompanying gaskets can fail, causing a leak.
On rare occasions, a very rich fuel condition smells like spilled gas. So, if all the above looks clean check your jetting. Whatever the case, all of the above affects the performance of the bike. It's likely you'll notice problems with the bike first, then smell the fresh gas. That should help narrow your focus when troubleshooting.
If oil is the blood of your dirt bike, gas is the food that keeps it running. And just like the human body you can have problems at the mouth (gas tank opening) to the stomach (gas tank), into the digestive system (engine) all the way until its pooed out (exhaust pipe). Bottom line? Check every gasket and function of the fuel system. If you smell gas, the problem is there somewhere.