Clunk from engine shut off help

Hi,

For the last 3-4 days when I finish riding my scooter (Sym Jet 4 125cc) and turn the engine off I've heard a weird sort of clunk sound when the engine turns off. The first time I heard it I thought something had fallen off my bike but didn't see anything and thought nothing of it but it now happens every time I switch off the engine.

This hasn't happened before now and not sure how to describe the sound, its like 2 bits of metal knocking against each other.

Scenario just be clear is I ride my scooter, come to a complete stop for parking, turn key to switch off engine, engine stops and I hear a clunk noise.

My scooter itself is still working perfectly fine its just this noise that's suddenly started.

Welcome to TMBF mate. .......pop over to the Newbies section and introduce yourself to the guys when you can. ......

Try and catch the noise on video and post it up to see if anyone can come up with any explanation. ......

Maybe a centrifugal clutch issue?

Thanks for the replies.

The sound seems to have gone now, haven't heard it in a couple of days. Hopefully it was nothing serious and whatever it was has sorted itself out. Weird.

Things don't just sort themselves out whatever was happening is likely to return when conditions are the same.

I am put in mind of the starting characteristics of old brit bikes. When kicking these over the engine was rotating so slowly that the mixture could ignite before the piston could get beyond top dead centre the engine was then driven in reverse direction and a massive kick back occurred. the most powerful bikes had ignition retard controls that minimised this effect. But any engine can be prone to this happening when rotating very slowly due to the mixture being timed to ignite before top dead centre.

Additionally as an engine is used the plug can gather carbon deposits if the mixture is over rich which then glow with residual heat and can cause premature ignition due to this residual heat burning the mixture on contact as the piston rises and the mixture becomes easier to ignite. The preignition of the mixture would happen even earlier than the spark plug would ignite it and the effect would be more pronounced.

I would hazard a guess that as the engine was dying the mixture was being ignited by glowing carbon deposits and the engine kicked into a reverse rotation just before stopping. The reversal of direction would sound very similar to that which you are describing.

I would remove and clean or replace the spark plug to minimise this risk and have the mixture checked. As you remove the spark plug inspect it, if it is covered in black sooty deposits then this could indicate the cause is as described. It is by no means conclusive however.

This is confirmed by the fact that the problem cleared up. As the weather changes and becomes colder an over rich mixture can become better suited to the environment and slightly harder to ignite, this difference could make the symptom disappear. The fact that the problem cleared up links it to environmental changes and different mixture needs.

Also during the symptoms a lot of short journeys would tend to increase sooting of the plug and glowing carbon deposits. Whereas a long journey would bring the plug up to working temperature and burn off those sooty deposits ending the problem.

Did you take the bike for a longer run immediately before the problem cleared up.