On a two-stroke engine, is the exhaust mixture in the pipe pure exhaust?

Does air that has not gone through the combustion camber of the engine some how get sucked into the expansion chamber of the pipe?

If I were to try to get a air/fuel mixture reading with an O2 sensor would it be an accurate measurement of the mixture that was spent in the combustion chamber? If so, where would be the most accurate place to take this reading?

Yes, non combusted air/fuel mixture does go through the pipe. I believe it is on the order of 30% depending on the bike. That is the reason two smokes are considered "dirty" They don't have a valve that shuts off the incoming air mixture, just a crude one (Reed valve) that keep exaust gasses from getting pushed back into the intake tract.

Socal. Thanks for the reply. But maybe I am not explaing myself.

I understand that there will be unburnt air/fuel in the expansion chamber, but because some two-stoke pipes do not have solid conections, ie. spings holding them together at the joints, does excees air enter the pipe that would change the air/fuel ratio from the actual ratio that entered on the intake side of the engine.

For example...

Lets say that my carb was setup to introduce a air/fuel ratio of 14:1. After this mixture has traveled through the intake ports, been fired at the top of the stroke, and pushed out into the pipe as exhaust, would this ratio still be at 14:1, or would there be an increase amount of air, lets say 18:1 (this might be an exageration) because of air that has been pulled in through the spring joints at the pipe or some other "leak"?

I understand you now. I don't have a clue. I would assume there would be some leakage as the connection isn't totally sealed but can't say for sure. Dirtrider.net should have something on the subject. There are some scary smart people on there. PM Rich Roerich if you join. If not, you should be able to find something out using the search function. Rich will be able to tell you exactly what you want to know.

Someone explained to me once that on a two stroke, air input = air output.
What comes in, must come out. Air in the exhaust is pure exhaust as long as the motor is running.

The amount of Oxygen molecules entering the intake compared to the molecules of oxygen exiting on the exhaust side may not be the same due to the burning effect of the spark.

The only time you can have air sucked through the exhaust is right at the point you shut the engine off.

Although the exhaust pipe is not bolted (it uses springs instead), it should be completely sealed to achieve the best compression as predetermined by the the expansion chamber of the pipe.

To answer your question: almost but no. On a properly designed expansion chamber the piston will cover the exhaust port just before is starts sucking the old exhaust in.

The reason for bad gas mileage on two strokes is because it’s better to strand a small amount of un-burnt fuel than to have exhaust gases reenter the cylinder before the piston closes the port. Of course in a perfect world you would get peak horsepower and fuel efficiency if the piston closed off the port as the last molecule of fresh fuel reentered the cylinder but that isn’t going to happen so they get it as close as they can.

Here’s a diagram:




And as stated previously the pipe to cylinder connection is sealed.