Knock in engine

Hi guys! I've a problem with 4th Gen VFR. When I'm turn a throttle on idling very sharp I've heard a very loud single knock. I think that it is comes from engine. But on idling without throttle everything is fine, when running also no problems. Please give a advise about this problem!!! What it could be?

 

Does it cough through the carbs, stall or stumble when you do this?

 

No. No coughing, no stalling or stumble.. no other sympthoms. 35 k miles on odometer. It starts week or two ago when i change the oil.
I bought this bike not long ago and i didnt know what oil was there. I've change old oil by Motul 7100 fully synthetic.
Could it be piston pin, because new oil is more liquid than old one in my opinion?

 

It would be the other way around...as oil gets "used" the viscosity breaks down, therfore lessening its effect on properly lubricating the system..... something like that...

 

This problem resolved by replacement an old one spark plugs. I think there was a detonation.. Knock is disappear now) I'm happy)