* Initial Warm-up: A cold 2-stroke engine has thicker oil, which increases internal friction. This friction resists the engine's ability to reach its maximum speed. As the engine warms up, the oil thins, reducing friction and allowing the engine to rev higher. The initial 4500 RPM reading is likely the engine's limited performance due to this cold, thick oil.
* Thermal Expansion: As the engine components heat up, they expand slightly. This expansion can subtly affect clearances within the engine, potentially leading to slightly improved performance in the short term.
* Fuel/Air Mixture: The carburetor (or fuel injection system) might be slightly out of tune, causing a richer mixture when cold and a leaner (more optimal) mixture as the engine warms. A slightly leaner mixture can allow for slightly higher RPMs.
* Second Attempt Decrease: The slight decrease in maximum RPM on the second attempt could be due to several factors:
* Heat Soak: The engine components have reached a more stable operating temperature. While expansion initially helped, excessive heat can lead to slightly increased clearances, and less efficient combustion.
* Fuel Depletion: If the initial high RPM run was sustained for a while, it may have slightly depleted fuel from the carburetor's bowl or lines, impacting subsequent performance.
* Measurement Inaccuracy: The simple act of measuring maximum RPM isn't perfectly precise. Slight variations in how the measurement is taken can lead to differences between attempts.
* Engine Condition: General wear and tear, or issues like a worn piston ring or ignition system problems, could also contribute to inconsistent RPM readings. A less-than-perfect engine might show more variability in performance based on temperature.
In short, the initial increase is likely due to reduced friction from warming oil, while the subsequent decrease points to the engine settling into a stable, albeit slightly less optimal, operating temperature. The variations could also stem from measurement error and the overall condition of the engine. If the RPM variations are significant or consistently problematic, further diagnostics might be needed to rule out underlying mechanical issues.
