I have a 2012 Suzuki GS150 R. It has done ~25200kms which I bought second hand 3 months ago. The problem with it is that it backfires(popping sound) when i take the bike above ~5-6k rpm and suddenly close the throttle. Also the bike is over heating after like 10-15mins of ride. On the brighter side, mileage is great, I am getting around 50kmpl.
I know running the bike lean will damage the internals of my bike, so I took it to my trusted mechanic he has finally given up. The following were the list of things that he did.
1. Tried to tune the AFR screw and no matter how rich he made, the bike kept backfiring - As far as i know AFR screw tunes the pilot jet. So I guess the pilots are in good shape
2. Cleaned the carburetor.
3. Checked the spark plug and it was ash white - So, the bike is running lean.
4. Cleaned the air filter.
5. I tried pulling the choke and revved in neutral gear, and just like I guessed, no backfiring which is making me believe that it has something to do with the Air Fuel Ratio.

My mechanic says that everything is fine and the overheating is just because of the Shell Engine Oil that I am using but I really do not have a peace of mind. Also he said there may be some problems with the ignition coil.

I am taking my bike to a Suzuki ASC on monday. Just before that, I want to hear some inputs from my fellow members. What could possibly be wrong? Are there any ways to diagnose that I can do at home?

Bike: 2010 Suzuki GS150R
ODO: ~25200
Engine Oil: Changed 2 weeks ago, running Shell AX7 Semi Synthetic
Everything Else: Stock

Thanks in advance.