I have a 1973 Yamaha Enduro. Why are there two places for spark plugs for the...

[Huffdaddy]

...same combustion chamber? There are two holes that have threads and i can screw a spark plug into both holes. i was wondering what the advantage is to having two holes, or if you only use one at a time, because there is a bolt that screws in as well.

 

[gdwrnch40]

Two reasons,big bore two strokes can use a compression release or run an extra spark plug.Two strokes are notorious for fouling plugs.

 

[Dan W]

To use 2 plugs you will need 2 plug wires.
the reason for 2 plugs is better spark propagation, a bigger fireball.

 

[Jazz]

back in the day you could put two plugs in the head in case you were riding and fouled a plug. some bike's had a compression release valve that went in one of the holes as well slowed the bike with out braking there was a two finger lever you could use..