I've always disagreed with forums that can't read by guests. To the point that if I can't read the forum to see what it is like before joining I won't join.
I would suggest that the General Discussion area is left open to none members to read, that way you can at least get an idea about the type of people that inhabit the forum. The sections like
Oily Rag
For Sale
Wanted
Ride Outs/Bike Meets
Could be made so that they can only be read by members, but guests can see that they exist. I'm sure you get the idea.