Why is it difficult to start a scooter when cold but easy when hot?
3 Answers
The reasons why a scooter is difficult to start when cold but easy when hot are as follows: Improper or long-term lack of maintenance: Check in sequence whether the fuel level is sufficient, whether the spark plug electrodes are normal, whether the oil level is normal and whether the oil's rated operating temperature is higher than the actual temperature, and whether the air filter is clogged. Different grades of oil are suitable for different temperature environments. Using high-viscosity oil in low-temperature environments will make the engine work harder, thus preventing normal starting. Improper carburetor adjustment: Many beginners do not know how to adjust the mixture ratio and often turn the screw without understanding it. If the fuel injection volume is reduced, it will lead to difficulty in starting the engine when cold.
As someone who's been repairing motorcycles for over a decade, I often encounter this cold-start difficulty issue. The main focus should be on checking the carburetor - gasoline atomizes poorly in cold weather, making the air-fuel mixture hard to adjust properly for starting. Once the engine warms up and metal parts expand for better sealing, the problem disappears. Battery issues are another culprit - capacity drops in low temperatures resulting in weak sparks, and spark plug carbon buildup also causes weak ignition. Low compression ratio is more troublesome, often caused by piston ring wear leading to air leakage when cold. I once dealt with a case where the owner messed up carburetor adjustments while changing the air filter himself, making cold starts impossible. My suggestion is to first spray carburetor cleaner, then check cylinder compression if that doesn't work, along with battery voltage. For older bikes, pay special attention to valve clearance - even a few tenths of a millimeter difference can cause big problems.
Last month, my little scooter had the same issue. I had to push it for two kilometers in the morning and almost ended up late. Later, I found out it was a faulty EFI system coolant temperature sensor, which falsely reported high temperature when cold, leading to insufficient fuel injection. Actually, there are quite a few factors that can affect cold starts: fuel pump pressure relief causing low fuel pressure when cold; engine oil viscosity being too high, making it hard for the crankshaft to turn; spark plugs reaching the end of their lifespan with eroded electrodes leading to weak sparks; and one that many overlook—gasoline with excessive moisture due to prolonged storage. Once, I filled up at a private gas station and ended up with this problem; after two weeks, the cold start sounded like a tractor. The most convenient solution is to go to a repair shop and read the fault codes—it’s much more effective than guessing blindly. Also, remember to check the air filter; if it’s too dirty and restricts airflow, starting becomes difficult.
