Thank you very much for all the replies.

Here is what i've done so far:
1. I switched to new glow plug last weekend. And today was my 2nd run with it
2. I took off the heatsink head and sprayed some wd40 since i don't have after run oil yet.
3. I took out some case (i think piston case.. do not know what it's called...) and cleaned it as well. There are three holes on there. one square looking and the other two thin rectangle shape. After i cleaned this, i hear some squeaking sound from engine or pullstart. I can't tell... gosh... did i just make it worse??? (i think i switched the position of holes....)
4. I charge my ignitor after each day run.
5. air filter gets cleaned once every two day.

Hmm... Since it's night, i'll try again tomorrow. shoot, i gotta take out fuel from tank. Thank you again for advance.

oh yeah, one more thing. I used to have this problem, but not nemore

When the glow plug is taken off from the engine, the pullstarts can be pulled very smoothly. BUT when it's on, it's hard to pull as i described in my first post. This problem is solved, BUT now it makes squeaking sound. i'm scared that i messed the whole engine.