i have a i beleive to be a 1980 GS550LX, and the plug that is in the side twards the bottom keeps blowing out when i reve up the RPMs. nothing excessively high just normal winding it out. is this a normal issue and if so how do i fix it. also i have one of the four carbs is cracked and would like to replace it how much should i pay for an aftermarket carb for it?