Engine popping when accelerating

I have a 98 Civic EX 1.6 VTEC I just finished doing the timing and I had to change 2 valves on the exhaust side, I just put a new cat on the car even. I now have a popping sound (kind of sounds like a backfire) when I accelerate and I do not have full power when I do accelerate even when I have my foot to the floor on the gas pedal. I am at a complete loss when it comes to fixing this civic. The popping sound seems to be coming from the muffler towards the back of the car. When the RPM are at 900 my timing is at 16 Degrees I just put a new aftermarket cat on the car and it is definately burning oil. it is blowing what looks like gray smoke. I started to mess with the car because it did not run right since the car was given to me to pay off a debt..


Engine Speed 1145 RPM
Ignition Timing Advance 20 Degrees
Manifold Absolute Pressure (MAP) 11.5 in Hg
O2 sensor – Bank 1 Sensor 2 (mV) 1015 Mv this sensor is constantly over 1000 Mv
O2 Sensor – Bank 1 Sensor 1 (mV) 505 Mv

You replaced two valves so you had the head off?
It really sounds like a valve train problem, either something
messed up in the V-tec or the valve train itself.
Are you sure the valves are adjusted right?
And that everything is reinstalled correctly?
You may want to pull the valve cover and recheck
everything.
First double check your firing order to be sure two plug wires aren't swapped.
And double check your cam tining, if the belt wasn't installed correctly,
it will do just that. So double check your timing marks.
The timing belt would be my first guess and check.:cool: