My engine pops once when I turn key

I have seen this about 5 or 6 times now. When I get on the bike after sitting overnight I turn the key on and the engine give a little pop and the engine turns once. It is strange to say the least. I do not hit the starter button when this happens. It sounds like it fires but it just might be a compression release sound. Does anyone know anything about this?
It runs great so I don't think it is anything major.

 

Mine had it too. When you turn the key the ignition is turned on which induces a spark in the cilinders. This ignites a tiny amount of fuel which is leaked into the exhaust. You could check if there is an air leak in the exhaust. That was the case @ my bike. The collectors tend to go bad trough the years.


grz Thijs

 

Only time I saw this was when I was flooding. I would also check your petcock and needles, could have a small leak. Check your oil for gas too.

 

Thanks I am having problems with my gas vacuum valve. I rebuilt it but it leaks. I have to take it apart and see what is going on with it. I think it my need a new spring. Thanks for the info!