How to fix: Fuel/oil mix ratio (normal on 2-strokes): how do you go about it?

On a two-stroke, a light haze of blue smoke is normal -- the mixing oil is meant to burn along with the fuel. What we're confirming here is excessive blue smoke -- thick, constant, with oil spitting from the exhaust -- which points to too much oil in the mix. The fix, in principle: bring the oil-to-fuel ratio back to what the manufacturer specifies. On a manual-mix 2T, that means being scrupulous about the ratio every time you fill up (often somewhere around 2% to 5%, i.e. 1:50 to 1:20 depending on the machine and oil -- check your manual). On a 2T with separate lubrication (a metered oil pump), it comes down to checking and adjusting the pump cable, which needs to be synchronized with the throttle per the manufacturer's timing mark. Risk level is low as far as the work goes, but be careful about which way you err: too much oil just fouls the plug and pipe (annoying but not serious), while too little oil starves the engine of lubrication and can seize the piston (an expensive failure). When in doubt, stay slightly on the rich side. On budget, this is essentially free -- it's a matter of method and ratio. A good quality 2T oil (semi-synthetic or full synthetic, correct spec) runs roughly 10 to 25 euros a liter. If the smoke stays excessive after correcting the ratio, suspect a crankshaft oil seal on the case side letting gearbox oil through -- at that point, you're looking at a bigger job. This tool helps you understand what's going on -- it doesn't replace a certified mechanic.

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Blue smoke from the exhaust

Bluish smoke — sometimes gray and a bit oily — comes out of the exhaust. In almost every case, this means a small amount of engine oil has gotten into the combustion chamber and is burning along with the fuel mixture: oil produces this characteristic blue tint, while excess fuel would give you black smoke, and coolant would give you white. Telling your engine type apart matters a lot here. On a two-stroke, oil is deliberately mixed with the fuel (manual mix) or metered by a pump (separate lubrication) to lubricate the bottom end: a light haze of blue smoke, especially when cold or when you roll on the throttle, is completely normal and nothing to worry about. On a four-stroke, oil and fuel are entirely separate systems: any persistent blue smoke is abnormal and points to internal wear. Signs that go along with real oil consumption: the level dropping between oil changes, an acrid, oily smell at the exhaust, a black and wet spark plug electrode, oily deposits inside the muffler. On a 4-stroke, when the smoke shows up strongly points the diagnosis in a direction: if it only appears at startup and then clears once the engine is warm, it's usually the valve stem seals; if it's there all the time and gets worse under acceleration or when you back off the throttle after a long downhill, it's more likely the piston rings. Urgency threshold: as long as you keep an eye on the oil level and top it up, you can keep riding, but consumption above roughly 0.5 L per 1,000 km calls for a prompt engine diagnosis to avoid fouling the catalytic converter and, eventually, engine damage.

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Possible causes: blue smoke from the exhaust

These are the most common causes, from most to least likely. The guided diagnosis above helps you find yours, test by test.

Fuel/oil mix ratio (normal on 2-strokes)
Basic
A two-stroke engine has no oil sump or separate lubrication circuit like a four-stroke does: special 2T oil is mixed into the fuel, either by hand in the tank (premix) or injected automatically by a pump metered to throttle position (separate/oil-injection lubrication). This oil lubricates the crankshaft, piston, and cylinder, then burns along with the mixture and exits through the exhaust. That's exactly why a light haze of blue smoke — especially on a cold start and when you back off the throttle — is completely normal on a 2-stroke and isn't a fault at all. The typical mix ratio runs between 2% and 5% oil (roughly 1:50 to 1:20), but the exact figure depends on the manufacturer and oil type, so check your manual. Excessive, thick, constant smoke, with oil spitting out of the pipe, points to a mix that's too oil-rich (a mixing error) or, on oil-injection systems, a pump whose delivery is set too generous or whose cable is poorly adjusted. On the other hand, a mix that's too lean on oil is far more serious: it causes inadequate lubrication and can seize the piston. If in doubt, it's better to run slightly too much oil (a bit more smoke) than too little. This is the first thing to check on a 2-stroke — it's free and needs no disassembly.
Learn about the part: Two-stroke premix →
Worn piston rings
Expert
Piston rings are metal rings seated in grooves cut into the piston; they seal the gap between the piston and cylinder wall and scrape oil off the walls to send it back down to the bottom end. A 4-stroke typically has three: two compression rings and one oil control ring. With wear (high mileage, repeated overheating, neglected oil changes, hard cold starts), the rings lose their tension and sharp edge, or gum up in their grooves due to carbon deposits: they no longer scrape oil properly, so it works its way up into the combustion chamber and burns. The result is blue smoke that's present all the time, and that clearly gets worse under load when accelerating (climbing a hill, wide-open throttle) and when you pick the throttle back up after engine-braking down a long descent. This cause is typical of a worn four-stroke engine; the term 'rings' also exists on a 2-stroke, but the diagnosis differs there. Accompanying signs: noticeable oil consumption (frequent top-ups), measurable compression loss on a compression tester, harder cold starts, and sluggish acceleration. Telling rings apart from valve seals matters: rings smoke all the time and under load, while valve seals mostly smoke at startup. Confirmation comes from a compression test and a leak-down test, and the fix requires opening up the engine — this is shop work (expert-only).
Repair to be left to a professional.
Learn about the part: Piston rings →
Worn valve stem seals
Expert
On a four-stroke engine, each valve slides in a guide lubricated by oil from the cylinder head. A small elastomer seal (the valve stem seal, sometimes called a valve guide seal) caps the top of the guide and limits how much oil works its way down the stem toward the combustion chamber or exhaust port. Over time, with heat and thousands of opening cycles, these seals harden, crack, or lose their sealing lip. With the engine off and sitting, oil slowly pools along the valve stems and seeps into the chamber. On the next start, this pooled oil burns off all at once: you'll see a distinct puff of blue smoke in the first few seconds — even when restarting after stopping at a light — and then it clears once the engine is warm and running at a steady speed. This is the tell-tale sign that separates this fault from worn rings (which smoke continuously and under load). It's mostly seen on 4-strokes with high mileage or that have sat unused for a long time. Oil level drops modestly, and the spark plug electrode may be wet. Replacing valve stem seals requires pulling the cylinder head (or at minimum the valve springs, using a dedicated spring compressor tool): this is a job for a professional (expert-only) — not to be confused with a simple adjustment.
Repair to be left to a professional.
Learn about the part: Valve stem seals →

Fuel/oil mix ratio (normal on 2-strokes)

Worn piston rings

Worn valve stem seals