If it is a light mist then this is normal.
Back in the old days the exhaust valve had some sort of low speed valve lifting mechanism built into it so you could pull start the engine and latter so very low powered starter motorc could turn it over fast enough to start.
Enter the shiny bum expert who decided the entire universe will cease to exist and every one will have 3 headed children with purple blood if the tiniest drop of unburned "fuel" ends up in the atmosphere.
So the decompress had to be moved from the exhaust ( where it worked very well ) to the inlet where it works marginally well.
So what you are seeing is the decompressing of the charge blowing back through the carb.
If your valve lash has become way too big, this will be quite pronounced because the incoming charge will be almost fully compressed so blow back through the carb quite fiercely.
Even more confusingly some engines will only have the decompression on one cylinder.
Add to that the fact that modern fuel ( which is nothing like petrol ) is very conductive at cclinder compression pressures you have the ideal situation for the spark running down the electrode to earth rather than umping the gap.
And just when you thought things could not get any worse the plug makers stopped glazing the center electrode porcelain insulator so once it starts to arc it is all over red rover.
I have had brand new plugs work perfectly when I put them in then fail to fir when the customer comes in to collect the engine, ( very embarrassing ) so I had a think and now never run an engine for any less then 15 minutes if it has new plugs in so the insulator can get up to full tmperature and prevent the oils in fuel ( which is not petrol ) depositing on the plug.
Once a plug is fuel fouled the only thing that will recover it is to burn the deposit off by heating up the plug with a torch till you get a yellow flame emerging from the center of the plug.
Record burn time is 13 minutes. And that ws on a brand new clean looking but wet plug.
