Hi Dave,

I don't really know the mileage to change the plugs.  I'll look it up in the owners manual and get back to you.  My reasoning is:  I have been chasing the rich mixture, not holding closed loop, surging and irregular idle, high idle problems with Baby for a couple of years.  Running rich I know from aircraft engines can foul the plugs.  I'm sure my first vette tech replaced them when I bought the car.  He also replaced the master cylinder and sand blasted the rotors as well as replacing one caliper.  That was 7 years and 16,000 miles ago.  I figured I'd find some carbon on the electrodes but, the old plugs looked pretty good.  The gaps were all larger than the spec and R45TSs are cheap.  So rather than just pulling the plugs and regapping them, I decided to just put new ones in.  I would expect 30 to 50 k miles if the plugs do not fail for another reason.  i.e., cracked insulator, burned electrode, stuff like that.  The more experienced guys might chime in on this. When pilots suspect fouled plugs, we lean out the mixture and do an extended runup on the ground or if in the air lean it out till it runs a little rough - enrichen slightly and just let it crank for a while so, we can plan where to put down if things go to $h_t.  But, you knew that.

Having fun in the Sun!
Keith