No problem, mate.
Have you checked the spark plug? If you have backfire it might be that your fuel mixture does not burn completely in the chamber and some goes out where it explodes. Could be that the mixture is rich. Play a bit with the Kev's mod. This might help. The bike should work smooth without PCV.
If this doesn't cure the problem, i am afraid, it might be that an exhaust valve is not closing completely letting some of the mixture out. This will be a tough job.
Keep us posted.

I would check the valve clearings also.

Valve clearings have been checked by a mechanic about 1000km ago so that should not be the issue..

My engine was taken apart because had a bad main bearing.. but it was backfiring before that also. Mechanic told me that in the engine everything looks almost new and up to specs.

I have a new iridium spark plug.. maybe tomorrow i will put that in and see if that helps.

Backfire comes almost always only when i close the throttle quickly or when engine braking. If i slow down very slowly and not close the throttle completely then it usually doesnt backfire.. Also when the bike is cold it doesnt seem to backfire due to a cold exthaust.

Anyways thanks for your help mate and i will keep you posted.

After you mentioned i might be running too rich i decided to put CO value back to stock (4). Dont know if previous owner had changed it but that what it was when i got the bike.. Anyways seems to pop and bang less, but still not as good as i would like it to be. Havent really tested properly yet but seems so.. After this weekend has gone its going to be tested properly :D

I would have changed the plug but cant find it anywhere so have to wait until it pops up or get a new one..

You can try with CO on zero and play a bit with the Kev's mod to make the mixture poorer, try few settings and follow the results

assenvas is your metalmule sticker still on the muffler? Mine burnt off after ~250km, might be due to exthaust wrap so its hotter than normal..

Will play with kev mod during weekend.. Now its set -5 degrees..

Still have the sticker but I do not know how long it will survive :D

My Metal mule fitted with out issue...
But I have dented the under side of the
muffler with the rear brake caliper under full
wheel travel...
O well I surpose it now has clearance...

I fitted my new Metalmule 2-1 system yesterday. I must really say that it was a pain in the a**. The front link pipe had a wrong angle towards the "2in1 link pipe" So it wouldn't fit without swearing and hammering ~8hours..

Well.. now it's there but it makes a strange noise, my guess is that i have a leak somewhere.. maybe where it connects to the cylinder. It might also leak where the front pipes connect together.

I'll take my bike to my local Yamaha dealer and they will tell their opinion.

Sika I've just replied your PM!