Exhaust fumes in the helmet or around the bike

You smell burnt exhaust gas (sharp, irritating) while riding or at a stop, in your helmet or around the bike. Warning — take this seriously: exhaust gas contains carbon monoxide (CO), an odorless and toxic gas, along with other irritating compounds — prolonged exposure, especially in an enclosed space like a garage, is dangerous. Never run the engine in a closed space. The most common cause is a leak in the exhaust line BEFORE the muffler: a manifold gasket, a clamp, or a cracked pipe lets gas escape near the engine, and the smell rises up toward the rider instead of being carried out behind the bike. On a 4-stroke, a melted or broken catalytic converter also lets more harmful gases through, and a failed lambda sensor (or an overly rich mixture) increases unburned hydrocarbons and the smell (which then leans closer to raw fuel). You can tell them apart by timing (smell that rises while riding = leak; at a stop with a hot engine = catalytic converter/richness), the type of smell (raw fuel vs. sharp burnt gas), and a visual inspection of the exhaust line (soot marks around a joint). Urgency threshold: an exhaust leak isn't mechanically serious, but exposure to toxic gas means it should be addressed quickly, and you should never expose yourself to it in an enclosed space. A manifold leak also throws off the mixture and can overheat a valve. Always ventilate, and get it repaired without delay: when the smell shows up, its type (raw fuel or burnt gas), and soot marks around joints are the three clues that will help you pin down the cause.