Oil leak

On the early model years of the KTM/Husqvarna 790 and 890 twins (790/890 Adventure and Duke, Norden 901), the valve-cover gasket is known to weep prematurely: a light oil film appears at the valve-cover sealing face, sometimes with a few drops on the crankcase or exhaust (a burning smell). As long as it's just a seep, it isn't serious, but it's worth treating to stay clean and avoid oil landing on the exhaust. Replacing the valve-cover gasket (and respecting the torque) is a job accessible to a careful DIYer. Also check the breather and the tightness before concluding.

This is the most common and simplest cause of an oil leak, especially when it appears right after an oil change. The drain plug seals the bottom of the sump with a gasket (aluminum/copper washer or O-ring) that crushes with each tightening — ideally it should be replaced at every oil change. The oil filter (screw-on or cartridge type) seals with an O-ring coated in oil. Classic mistakes: insufficient torque, a reused gasket already crushed, a double gasket on the filter (the old gasket stuck to the mount, with the new one placed on top of it — a common cause of a clear leak), or conversely over-tightening that deforms the gasket or strips the aluminum case's thread. The result is seepage, sometimes a drip, under the engine. This is a benign cause, easy to fix: retighten to spec with a new gasket, or redo the filter's seal. This is an easy DIY job. The tell: a leak that appeared right after an oil change, located at the drain plug or the filter. Important: follow the manufacturer's torque spec (don't force it into aluminum), use a new gasket, and check there's no double gasket. After the repair, clean up and watch for any further seepage.

The engine is closed off by several covers (cylinder head cover / rocker cover, ignition cover, clutch cover, various other covers), each sealed with a gasket (paper, elastomer, O-ring, or sealant). With heat, thermal cycling, and aging, these gaskets harden, get crushed, or crack, and oil starts to seep — leaving greasy traces on the block, sometimes drips on the ground. This is a common cause on aging motorcycles. The leak is generally moderate (it soils more than it drains), but a significant leak can lower the level, worth monitoring. The cylinder head cover is a classic culprit (a gasket that hardens, oil running down the cylinder and exhaust, with a hot smell). The tell: oil traces starting from around the perimeter of a cover, on the lower engine. The fix principle: remove the cover, clean the gasket surfaces, and fit a new gasket (or the appropriate sealant) to spec torque. This is an intermediate job, with difficulty depending on how accessible the cover is. This cause overlaps with the dedicated case/cover leak symptom, and with oil consumption (an external leak lowers the level).

A shaft seal is a lip seal that seals oil around a moving shaft or tube. Two important locations. Warning: the FORK shaft seal seals the fork oil around the tube; when worn, it lets oil run down the tube, then onto the caliper, disc, and pads — this is a braking DANGER (an oiled disc or pads no longer brake properly), to be addressed without delay, and the brake must absolutely be cleaned. You spot it by an oil ring on the fork tube above the front wheel. The gearbox OUTPUT shaft seal (engine) seals oil around the output sprocket shaft; when worn, it lets oil seep onto the sprocket and chain. The tell: oil on the fork tubes/front wheel (fork seal) or on the output sprocket (engine seal). The fix principle: replace the seal in question — for the fork, remove the tube, replace the seal and the fork oil (an intermediate job, but affecting the front end and brakes, so rigor is needed); for the gearbox output, remove the sprocket, more technical depending on the model. This cause overlaps with the transmission oil leak symptom. If you're not equipped/comfortable, especially for the fork (safety), leave the job to a professional.

Rarer and more serious, a degraded head gasket can show up as an oil leak between the head and cylinder (external seepage at the junction), or as internal cross-communication between the oil, coolant, and combustion chamber circuits. The head gasket separates these circuits, which must never mix; when it degrades (often after overheating that warped the sealing surface), oil can seep externally, or mix with the coolant (a beige "mayonnaise" emulsion under the oil filler cap, coolant level dropping), or even burn in the combustion chamber (smoke). The tell: a leak located at the head/cylinder junction, or signs of oil-water mixing (mayonnaise, overheating, white smoke). This is a major engine failure: the repair requires removing the head, checking/machining its flatness, and reinstalling to spec with a new gasket — a job reserved for a professional (expert-only). To distinguish from benign external leaks (plug, cover): here, either the leak is at the head/cylinder junction, or it comes with internal signs (mayonnaise, fluid loss, smoke). Don't ignore these signs, since riding with a failing head gasket quickly worsens the damage. This cause overlaps with the white smoke and overheating symptoms.

Repair to be left to a professional.