Why does the engine lack power after a major overhaul?

[Ryan]

After a major engine overhaul, where components such as the crankshaft assembly, cylinder block, pistons, and piston rings are replaced, the vehicle's power performance is usually better than before the repair. However, some engines may experience symptoms such as sluggish acceleration, poor power performance, and high fuel consumption after repair. The main reasons are as follows: 1. The quality of the replaced parts is poor. Some crankshaft assemblies have insufficient concentricity between the left and right shafts, exceeding the standard (0.15mm); the distance between the left and right crankshaft bearings exceeds the standard by 1mm; the intake and exhaust ports of a two-stroke engine cylinder block deviate more than 1mm up, down, left, or right; the inner diameter of the cylinder exceeds the standard, and the surface of the cylinder wall is roughly machined; some inferior piston rings are made of steel wire, with poor elasticity, wear resistance, and high-temperature resistance. The cylinder block's sealing performance decreases, causing gas leakage, and the engine's cylinder pressure drops, affecting power performance. 2. The repair and assembly process is not standardized. For example, when disassembling the crankcase, instead of using special tools (such as a case splitter), methods like hammering or prying with a screwdriver are used, resulting in deep scratches or deformation on the crankcase mating surface, causing air leakage. After the left and right crankcases are joined, the concentricity of the crankshaft is poor, increasing friction between moving parts, causing the engine to overheat and reducing power performance. 3. After the vehicle is repaired, the engine's lubrication system should be checked. The process of replacing the cylinder block and crankshaft requires cleaning the old cylinder gasket. If the cylinder gasket in the crankcase is not thoroughly cleaned, the oil passages can easily be blocked by debris such as gasket material or sealant, preventing the lubrication system from functioning properly. Due to poor lubrication, the wear of components such as the camshaft and rocker arms in the valve train intensifies. The wear of the camshaft and rocker arms affects the opening height and closing time of the intake and exhaust valves, reducing the cylinder's air intake and decreasing the motorcycle's power performance.