Blowing
oil out the various orifices is to easy.
You need to vent the engine better. While the pros and cons of a Positive Crankcase Ventilation (PCV) valve have been debated at length in various threads, I STILL don't know what the "fuss" is all about.
In my opinion it's just plain COMMON SENSE to run a PCV valve or SOME POSITIVE method to VENT the internals of the engine. A "breather" on the valve cover OFTEN just isn't enough!
I am unclear as to your "back fire" condition. Is that a "popping" sound you get on deceleration in the exhaust? If so, many there are several potential different causes. Usually an exhaust leak allowing fresh air in mixing with the exhaust. Not a "big" issue unless the "noise" bothers you, won't hurt anything.
Now BACK FIRE is through the carb (or intake in you would). Timing and ignition is the most common cause. A WEAK spark can cause it. Check the ADVANCE of the timing, not JUST the base timing. A "lean" condition will cause it (lean could be AIR leak at the carb or manifold).
Unlikely (and more serious) causes would include things like:
Cam profile/timing not right or valves hanging (not closing properly or on time).
Ernie