Motorcycle Repair: dirtbike, stock air filter, air fuel mixture

Question
i have an 1982 honda xl100s dirtbike when i ride it the carb seems to get to cold and it does at lot of poopping back what could be the problem

Answer
Elliott, When the engine is running, pressure waves surge in and out of the intake and exhaust system moving the air/fuel mixture past the venturi in the carburetor, mixing with the outside air on some of the exchanges. So, there is a wide temperature variation in parts of the engine.

Carburetor "icing" was a big problem a long time ago on old cars and motorcycles too, especially in colder temperatures.

The little 50-100cc bikes seem to be neglected a lot when it comes to maintenance of the valve clearances and ignition timing.  The valve clearances are only .002" but must be checked and kept adjusted, otherwise as the motor warms up the clearances decrease and the engine loses power and some of the compression charge blows back out of the carburetor/intake. Make sure that the valves are adjusted properly at the T mark on the flywheel when the engine is on the compression stroke (both valves are closed).

Second, the ignition points, located beneath the engine flywheel/rotor must be adjusted so that they just OPEN when the F mark comes to alignment with the mark on the engine case. When the engine runs the point rubbing block slowly wears down and the points do not open as far and as soon as they are supposed to. This retards the ignition timing and that affects the carburetion, as well.

Give the bike a good tune-up, check the compression readings, adjust the valves and ignition timing and see if things don't improve a lot. If you ditched the stock air filter and changed the muffler, that will change the carburetion calibration, so you may need to put in a large main jet to compensate for the changes.

Bill Silver