Two Spark Plugs - One Cylinder

 I have a circa 1970 Yamaha 175 enduro motorcycle that has one
cylinder, but two spark plugs.  I've never had a spark plug wire on
the 'other' plug and it's run just fine.  Now, many many years later,
I'm trying to fix this bike up for my son and I'm wondering, "What the
heck is this other plug for???" I don't even think there is another
wire.
Thanks much for any help you can provide.

 
Are you sure it's not for a compression release?

Does it look at all like any of these?
http://www.jpcycles.com/groups.aspx?LID=ee35cd18-0412-4e15-86fb-215b8c7de029&DID=e71a7b64-ca6e-4f44-bb52-8bbde776554e

However, because the bike is so old, it may have just
been plugged somewhere along the line, without any
ignition wire or release cable.

 
It was there in theory so you could switch over to the "good plug"
once you foul the "regular" plug.
I don't know how it worked though I had a couple of bikes like that.

 

Someone years ago in some off road competition found that by replacing the
blanking plug with a second spark plug, they got better performance (and
probably better fuel economy). If you obtain a "wasted spark" coil - with 2
HT leads from a parallel twin (or 1 of the coils from a 4-cyl) you can fire
both plugs together and by all accounts I've heard, get a noticeable boost.

 

ian field wrote:

never heard this one [about using two plugs at once.

The compression release was used by 50% of the racers, and the 2nd plug 50%
by "other" racers.  They would use the 2nd plug in the desert incase the
first plug became fouled [too much oil mix].  The compression release came
in handy instead of using the brake(s).

--
Zyp