Funny...I also have a theory about why plugs fail: They're like fuses. If they blow, there's something wrong in your system or you're using the wrong heat range of plugs. If you're running the right mixture and the right heat range of plug, any commercially available $2 spark plug will last thousands of miles. I ran one set of B7ES plugs in my Royal Star for 18K miles until one of the coils started to go out. The only reason I replaced the plugs was because they were the easiest thing to replace once I experienced a misfire. My '69 VW hasn't had the standard Bosch copper plugs changed in at least three years and 22,000 miles.
The only times I've had plugs fail is when a) they got fouled or b)I had excessive pinging. The first instance is probably the most common failure mode because the conventional wisdom in Britbike circles is to run too rich. Set the idle too rich because the choke slides have been removed...run too rich on the needle because the needle jet is worn...run too rich on the mains because you're afraid to hole a piston or you think more fuel will make your bike faster... I would say at least one of the above is true for 90% of vintage bikes out there. And then we wonder why plugs foul?
I did, however, once have an interesting failure on my golden flash (bear with me, there's an interesting punch line coming). When I was first getting to know the bike, I took it on a long ride in the mountains and it was pinging something awful. The combination of the DPO putting 8.25:1 compression pistons in it, my running too hot of a plug and the timing being too advanced resulted in the bike going on on cylinder during a particularly strenuous climb. On the downhill sections it would occasionally go back to firing both cylinders, but as soon as rpm's dropped below maybe 3000, it was back to a 325 cc single. After letting the bike cool down (it was VERY hot), I pulled the plugs and noticed that on one of the plugs, the electrode was MISSING. It had actually broken off. Replaced plug, took it easy on the uphills and got home just fine. As I said, timing turned out to be too advanced and the B6HS plugs were too hot for the circumstance.
I guess the moral of the story is: take the time to make sure your timing is right and your mixture is correct. The best way to do this, IMHO, is to read your spark plugs. I don't mean that garbage about "chocolate brown" in the chilton's manual, but a PROPER plug read. Here is a great place to start learning if you don't already know:
http://www.strappe.com/plugs.htmlCheers...
