A seized engine is when the crankshaft, rod, and piston are frozen solid and cannot move. You try to pedal start it and it acts like a brake when you pull in the start lever (on a manual starting clutch type), or when the automatic starting clutch engages. It never makes that chug chug chug sound of the piston going up and down.
Here are the steps to diagnose and/or recover from a seized engine.
1. Spray WD40 into the spark plug hole. This will get solvent to the piston top and cylinder wall above the rings. If the piston is just stuck with tar from sitting, this will dissolve some of the tar and maybe unfreeze the engine.
2. Spray WD40 into carburetor (after removing the air filter) with throttle held wide open. This will get oil into the crankcase and lower part of the piston and cylinder, below the rings.
3. Locate the flywheel. Some bikes it’s on the left and some it’s on the right. The magneto flywheel on a moped is a 4 or 5 inch wheel that has magnets inside it. Most have wide holes that allow adjusting the points. The flywheel is attached directly to the crankshaft. It is crucial to not confuse the clutch wheel (which turns with the rear tire) with the flywheel (which turns with the engine). Motobecanes and Peugeots have exposed “wheels” on both sides on the engine.
4. Try to rotate the flywheel with both hands. If it moves, try going back and forth, each time a little more, until it goes all the way around. Then spray more WD40 in and go around again many times. When it’s loose enough, switch to pedalling to rotate the engine faster than by hand. When it’s finally free, it will not feel tight anywhere (with the spark plug removed). A normal flywheel that’s “free” should move in a full circle with one strong finger. If it takes a whole hand then it’s “tight”, not “free”.
5. Determine what is stuck. A small screwdriver in the spark plug hole is used to feel the top of the piston. When just the piston is stuck, the crank will move a tiny bit back and forth, but the top of the piston will not move at all. When something like a loose screw or piece of debris gets stuck in the flywheel, the flywheel might move free within a range. Then the top of the piston will move with the crank. When neither the piston nor flywheel moves at all, then remove the flywheel and try to rotate the crank. If the naked crank is does not move, the engine is stuck internally, and the flywheel will then need to be put back on tight to continue the unfreeze process. If the crank and piston move free, then the flywheel is rubbing or sticking somehow. That can be caused by several different things each with a different remedy.
6. Find or make a tool to hold onto the flywheel, once you’re sure it’s stuck internally. Be careful to not allow the tool to damage the coils or wires behind the flywheel. Apply a rotation torque of, perhaps, 150 ft-lb for an 8mm crank nut, or 200 ft-lb for a 10mm nut crankshaft. These are estimates only. Be advised that too much torque can shear off the woodruff key and/or damage the crankshaft and/or flywheel woodruff key groove, or even break the crankshaft (possibly $500 damage). Applying high torque is risky but beneficial.
To be continued …