I have a 4 stroke 125cc chinese scooter which has lost 10mph top speed and has a loud rattle noise from front of engine at top speed. I have changed ignition coil, HT lead and spark plug, Oil change, Adjusted valves. Scooter starts and runs fine until top end is reached. Was 60mph now 50mph any help please.
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I can't find that model, but other models like the JCL MP-050B are 50cc four-stroke 139QMB engines. I'm assuming that's what your scooter has as well, since many Chinese scooters are renamed, but often keep a model number across various re-brandings. These engines have no way designed into them for the user to set ignition timing. You would have to modify/replace the pickup, flywheel, or CDI to change ignition timing. If you mean camshaft/valve timing, then you need to get the engine to TDC (top dead center) on the compression stroke. The cam has 3 holes in it's gear. One large hole and then a small hole 90 degrees away each way. At TDC on the compression stroke, the large hole should sit at the very top/front of the engine. The two small holes should be parallel with the top of the rocker assembly.
If it sounds like there's a ball bearing rattling around in your engine there probably is... take off the side cover and investigate it can't get any worse...if you keep riding it it'll only go bang and wreck the whole engine
It isn't a fuel problem, it is a power problem. Any bike will slow down on a hill. The bigger the engine, the more power is available to maintain speed. 50cc just isn't much power to pull a 150 pound man up a hill.
I am assuming the scooter has a two stroke engine. If it is a four stroke the valves may be needing attention. But even with good valves, 50cc just isn't a powerhouse of an engine. If you want to be happy, trade the scooter in on something 125cc to 200cc. You will feel better about your ride and be more respected on the roadway.