why do you think its the spark plugs? Are they fouled? What color are they- black, white, oily, burnt? If it bogs down, I think you have other issues than your spark plugs. Also, how tight are you putting them in? I'm not questioning your intelligence or engine knowledge, but I just found a spark plug in my bike that was supposedly serviced by a reputable place and they bent one of the plugs.

Just saying.