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I was wondering if it would be better to run 2 spark plugs in a double hole cylinder head or to use a metal plug?
Would having 2 spark plugs effect the compression? Because the spark plug takes up more volume then the metal plug, I think it would.
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Good point Mark, and I'm glad you mentioned that . . .
Don't be using a hotter spark plug range then what Yamaha originally recommended as being safe for your particular bike.
If your bike is overheating during prolonged hi-speed running with the proper spark plug heat range that Yamaha recommends, then you likely have other tuning problems ... such as ... over advanced timing, too lean on carburetion tuning ... or, perhaps an air leak. For this reason, I recommend that all riders learn how to properly "read" a spark plug ..... especially on these vintage two strokes.
Much more could be said about spark plugs, racing and tuning, but I hope we've covered your basic question. I might add that some have used the second hole to install a compression release, but that was more a thing of the past.
In my experience, if the first plug "just goes bad" then the second plug will probably be okay. If the first plug fouls due to a tuning issue for example, the second plug will likely be dead as well. So you never want to rely solely on the second plug in the head as your only spare.
I would not recommend two heat ranges... the spark itself is flea power in terms of heating the center electrode and insulator of the plug. The center electrode and insulator are "tuned" (heat range) to get hot enough to stay somewhat clean, but not too hot so they become incandescent and cause preignition when exposed to the blast furnace of combustion. "Hot" plugs have a longer heat path and "cold" plugs have a shorter heat path. How hot the center electrode and insulator get when exposed to the heat in the combustion chamber is not dependent on if the plug is sparking.
When I was young at the races it was somewhat common to warm up the bike on a "hot" plug and then swap to a "cold" plug for the race. One of the older guys in the neighborhood that was racing decided to run a really hot plug like a B5E or B6E in one plug hole for warm up, and then switch the plug cap over to the B9E or B10E for the race... he holed the piston very quickly after the start. That "hot" warm up plug was the first blistered/melted spark plug I ever saw. The cold plug did not look overheated at all.
That makes sense. I figured if Yamaha had a metal plug in the head from the factory, instead of a 2nd spark plug, there must have been a reason.
Hello, and welcome to the site.
I'm with Schu . . . it's more a matter of preference, but might depend somewhat on how the bike is going to be used.
If primarily used for racing, I could see myself only using one spark plug and plugging the other hole (especially if one of the plugs was more centrally located, and would thus encourage faster and more even burning during the combustion process).
The advantage of two plugs is as Schu said . . . as well as the option of having a one step cooler then stock plug present, for use during sustained hi-speed commuting.
I can't answer the technical question, but I would assume the effects would be minuscule. I have several of these old enduros and some have two spark plugs and some have steel close off fittings. My second unused spark plug is usually nothing more than a simple means of plugging up the second hole. But on a few occasions it has actually helped me get the bike started by switching the cap over to that second plug and it fired right up. Putting a brand new plug in that second hole just seems that it would carbon up and become useless. But evidently not! Anxious to hear from others.