Any reason why its such a large jet

My nrg mc3 isn't running the best a little bit on the spluttering side,i decided to change the the jet and seen it has a 92 in it.The bike has a 70cc kit some cheap make,original airbox with a malossi filter and a hebo exhaust and a 17.5 dellorto carb.
The standard jet on this bike is a 53,using all the upjetting guides that would put it about 74-76 jet but it looks like it needs upjetting again to a 94.
I have checked a few things over and there doesn't seem to be any leaks but have noticed that when started the exhaust seems to blow a lot of oil,carbon.
Any ideas

 

It should'nt need such a big jet, a 180cc 2t runner uses a 90 main jet. What's the plug colour after a good thrash?

 

Looking at the plug I would say its running really rich to the point its a very dark brown but if I try to drop down a size it starts spluttering

 

The plug need's to be a very dark chocolate, not black colour. If it's dark choc then you might need to look elsewhere.

 

Yeah it is a chocolate brown but I also notice there I a strong smell of petrol if the bike is left standing.

 

any leaks from the fuel tank or hoses? any drips around the carb float bowl?

 

No replaced the gasket on the drain screw and the bowl also changed the fuel pipe.I think its time for a full rebuild as I get like a rubbing vibration at about 20-30 when slowing down,also there is a knocking on tickover not from the variator side but the other side.The bike has had these problems since I had it so now I think its time for a rebuild,crank bearings,seals,head and carb also variator and clutch