Sumith, it sounds like your engine has more than one problem.

The spark timing should be set to a known value (like 10 BTDC) instead of just guessing, backing it off, advancing it forward. Because:
* static timing is the foundation of engine tuning. It should not be guessed at.
* it could be affecting the other problems. Once you have all the problems fixed, then backing it off, advancing it "by ear" would be OK for fine tuning.

I recommend you get a repair manual, RPM gauge and dwell meter (new $25 USD) -- and do it yourself. Then you'll know how it's done and won't have to rely on a mechanic who so far hasn't solved your problems. Just take it one step at a time, until all the settings are correct and the problems solved one at a time.