Based on the information you provided, it seems like there could be an issue with the fuel delivery system or an air-related problem in your Royal Enfield Classic 350. Here are a few possible causes that may explain the symptoms you described:
Fuel Delivery Issue: It's possible that there is a problem with the fuel delivery system, such as a clogged fuel line or a malfunctioning fuel pump. This can cause the motorcycle to stop midway or hesitate to pick up speed, as it may not be receiving an adequate supply of fuel.
Fuel Filter Blockage: A clogged fuel filter can restrict the flow of fuel to the engine, leading to similar symptoms. Over time, the fuel filter can accumulate dirt, debris, or contaminants, causing poor fuel flow and affecting the motorcycle's performance.
Air Intake Issues: The mechanic mentioned an air-related issue, which could indicate problems with the air intake system. This might include a clogged air filter or a malfunctioning air intake sensor. If the engine is not getting enough clean air, it can cause a disruption in the combustion process, resulting in poor performance and stalling.
Ignition System Problems: Issues with the ignition system, such as faulty spark plugs or a malfunctioning ignition coil, can also cause the engine to hesitate and stall during riding.
Given these possibilities, it's essential to have a qualified mechanic thoroughly inspect and diagnose the issue with your Royal Enfield Classic 350. They will be able to conduct the necessary tests, check the fuel system, examine the air intake components, and inspect the ignition system to pinpoint the exact cause of the problem. Based on their findings, they can provide you with the appropriate solution and perform any necessary repairs or replacements to resolve the issue.