After reassembling a two-wheeler SI engine following basic service inspection, which check is essential before returning the vehicle to the customer?

1. Omit the oil fill and run the engine briefly to check for unusual noises first
2. Verify engine oil level, clutch cable free play and that the engine starts and runs without obvious abnormal noise or leaks
3. Leave the spark plug lead disconnected so the engine cannot flood
4. Top up the crankcase with petrol if engine oil is unavailable