Adding a plenum or modifying the intake manifold on a single-cylinder carbureted engine can potentially impact its performance, but the outcome may vary depending on several factors. Here's some information to consider:
Plenum Function: A plenum chamber, commonly found in multi-cylinder engines, acts as a reservoir to store and distribute air evenly to each cylinder. It helps smooth out the airflow pulses and improves the volumetric efficiency of the engine. However, on a single-cylinder engine, the effect of a plenum is typically less pronounced because there is only one cylinder to feed.
Intake Manifold Modifications: Modifying the inner side of the intake manifold to increase its size can have some effects on the engine's performance. Enlarging the intake manifold can potentially improve the air intake flow and increase the volume of air available for combustion. This modification may result in a slight increase in power at certain engine speeds, particularly in the high RPM range where the engine is operating at its peak.
Carburetor Tuning: When making changes to the intake system, it is essential to reevaluate the carburetor tuning to ensure the air-fuel mixture is appropriately adjusted. The increased airflow due to the modified intake manifold may require adjustments to the carburetor's jetting, needle position, or air screw to maintain the optimal fuel mixture. Improper carburetor tuning can lead to issues like lean or rich running, which can adversely affect performance and potentially cause engine damage.
Overall Engine Design: The specific characteristics of your small 51cc, high-revving 2-stroke moped engine will play a significant role in determining the impact of the plenum-like modification. Factors such as port timing, exhaust system design, and the overall efficiency of the engine will influence how the modified intake system affects performance.
To assess the true impact of the modification, it is recommended to carefully monitor the engine's performance, including power delivery, throttle response, and any changes in the exhaust note or engine behavior. It's also advisable to consult with experienced motorcycle tuners or engine builders who have expertise in modifying and tuning small 2-stroke engines. They can provide guidance specific to your engine model and help optimize its performance while considering factors like air intake dynamics, fuel delivery, and combustion efficiency.