A two stroke is supposed to smoke. It burns oil mixed into the fuel on purpose, so a light blue haze, especially at idle and on a cold start, is the engine working correctly. What matters is the color, the amount, and whether it clears once the bike is warm. Thick white clouds usually mean too much oil in the mix. Black smoke means it is running rich. Sweet smelling white smoke that never clears is the one worth stopping for.
Read the color first
Light blue haze, clears as it warms up
Normal. This is burnt two stroke oil and it is what the engine is designed to do. You will see more of it at idle, more on a cold start, and more on the first tank of a new bike while it is breaking in.
Thick white or grey smoke that hangs around
Almost always too much oil in the fuel mix. The ratio for our bikes is 25 to 1, which is about 5.12 oz of two stroke oil per gallon of regular unleaded. Eyeballing it, or pouring in a whole bottle sized for a different ratio, is the usual cause.
Drain it, mix a fresh batch properly and run it through. How to mix 25 to 1.
It can also be an over oiled air filter. If you soaked the foam and did not squeeze the excess out, that oil gets pulled through the engine. How to clean and oil a filter correctly.
Black smoke
Running rich, meaning too much fuel for the air it is getting. Start with the air filter, because a clogged filter is the most common cause and the cheapest fix. After that it is the carburetor: the choke stuck partly on, the air screw well out of adjustment, or a float that is not shutting off.
The carburetor guide covers setting the idle and the air screw properly.
White smoke that smells sweet, and will not clear
This is the one to stop for. Sweet smelling white smoke means coolant is getting burnt, which on a liquid cooled engine points at a head gasket.
Our minis are air cooled, so they have no coolant at all and this should never appear on one. If you are seeing it on an air cooled bike, something else is burning that should not be, and the right move is to shut it off rather than ride it home.
When smoke is normal and you can ignore it
- The first few minutes of a cold start. Oil pools in the crankcase overnight and burns off once things warm up.
- Idling for a long time. Two strokes load up with unburnt fuel at idle and smoke while they clear it. Blip the throttle and it goes away.
- The first tank on a brand new bike. Break in is often run slightly richer on purpose. See how to break in a new two stroke.
- Cold, damp mornings. Some of what you are looking at is water vapor, not oil, and it disappears as the exhaust heats up.
What to check, in order
- The fuel mix. Was this batch measured or guessed? Guessed is the answer most of the time. Drain and remix at 25 to 1.
- How old is the fuel? Premix goes stale in about 30 days and burns badly after that.
- The air filter. Clogged means black smoke. Soaked in oil means white smoke. Either way, pull it and look.
- The choke. Push it fully off once the engine has caught. A choke left partly on will smoke and foul plugs all day.
- The spark plug. Pull it and read it. Wet and black is rich, dry and white is lean, light brown is right. How to read a plug.
- The exhaust. A silencer packed with old oil will smoke on its own long after the mix is fixed. Take it off and check whether it is wet inside.
The thing that causes most of it
More two stroke problems trace back to fuel than to everything else combined. Fuel measured by eye, fuel mixed weeks ago, fuel left in a carburetor bowl all winter.
Two habits fix most of it. Mix only what you will use inside a month, and turn the fuel valve off before you park it, letting the bike idle until it dies so the bowl empties.
Common questions
Is it bad if my 2 stroke smokes?
Not on its own. A light blue haze is the engine doing what it is built to do. Judge it by color, by whether it clears as the bike warms, and by how the bike runs.
Will too much oil damage the engine?
It will not seize it, but it fouls plugs, coats the exhaust, costs you power and makes it hard to start. Too little oil is the dangerous direction, and that one does seize engines. When you are unsure, err toward more oil, then fix the ratio properly.
My bike smokes and will not idle. Related?
Usually yes. Both point at the fuel side, either the mix or the carburetor. Work through the carburetor checklist and if it will not start at all, the will not start list.
Do electric dirt bikes have this problem?
No. There is no fuel, no oil and no exhaust on an electric, which is a large part of why people move to them. Our electric dirt bikes are the X5 at $975 and the X7 at $1,350.
New bike, smoking a lot. Is it faulty?
Probably not. A new two stroke on its first tank smokes more than it will later. If it is still doing it after two or three tanks of correctly mixed fresh fuel, get in touch.
Still stuck
Send us a short video with the engine running and the smoke visible. Text 623 320 8984. If a part came through defective, we take the claim to the manufacturer on your behalf and get the correct part sent to you free. Full maintenance walkthroughs are at manual.pluggedby50cc.com.
Plugged by 50CC is an independent retailer. We do not manufacture, assemble, modify or service the bikes we sell. Every bike ships sealed in its factory crate exactly as it reaches us, about 90% assembled from the factory, and final assembly and setup are completed by the buyer. Off-road use only. Riders under 18 should always be supervised by an adult.
Everyone should wear a helmet and gear.