Wolf - The EG series plugs are no more prone to fouling than the ES series. The fine wire EG plugs take less ignition voltage to fire and MAY be necessary in some bikes to account for the cost reduced ignition components that come stock. The advantage of the ES series plugs (besides cost) :
- they tend to be easier to read
- they tend to show color and heat on the ground strap and center electrode more quickly than the fine wire plugs. This makes plug reading much simpler and make the ES plugs the better choice for tuning purposes. Basic rule tune on ES race on the OEM recommended plug series.
Bikes that come standard with fine wire plugs usually need them IF they are run fairly hard. For more normal riding it usually won't make much of a difference.