There could be several reasons why a motorcycle might burn oil quickly after an engine rebuild. Here are a few possible explanations:
Piston rings not properly seated: During an engine rebuild, it's essential to ensure that the piston rings are correctly installed and seated against the cylinder walls. If the rings are not properly seated, it can lead to increased oil consumption as oil can pass through the gaps and get burned in the combustion chamber.
Cylinder wall damage: If the cylinder walls were damaged during the engine rebuild or if they were not properly honed or bored to the correct specifications, it can result in excessive oil consumption. Imperfections or excessive wear on the cylinder walls can lead to oil leakage into the combustion chamber and subsequent burning.
Valve seal issues: The valve seals play a crucial role in preventing oil from entering the combustion chamber when the valves are closed. If the valve seals were not properly installed or if they are worn out, it can cause oil to leak into the combustion chamber, resulting in increased oil consumption and burning.
Improper break-in procedure: After an engine rebuild, it's important to follow a proper break-in procedure recommended by the motorcycle manufacturer or engine builder. Failure to do so can lead to excessive friction and oil consumption. During the break-in period, the piston rings and other components need to settle and seat properly, which can affect oil consumption if not done correctly.
Overfilling of oil: If too much oil is added during the engine rebuild or during subsequent oil changes, it can cause excessive oil consumption and burning. Overfilled oil can lead to increased pressure in the crankcase, causing oil to be forced past the piston rings and burned in the combustion chamber.
If you're experiencing excessive oil consumption after an engine rebuild, it's best to consult a qualified motorcycle mechanic or the shop that performed the rebuild. They can diagnose the issue more accurately by inspecting the motorcycle and its components to determine the exact cause of the problem.