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1978 IT250 Spark Plug(s)

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Just picked up a '78 IT250 last weekend. The engine has 2 spark plugs on the top of it, one is connected, and the other has to plug connected to it. I was told that it was for a backup or something. I also hear that it's supposed to be for a decompression lever.

Anyone know? Thanks!

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Yes many people run two plugs in these engines but the one hole is suppose to have a bolt type thing in it. I'm not sure what the purpose of it is but many of the older yamaha two strokes had them. You could hook up a compression release or put a spark plug in it or find the oem plug/bolt thing. Here you can see how it was from the factory. http://www.yamaha-motor.com/partviewer/default.aspx?ls=sport#/Yamaha/IT250E_-_1978/CYLINDER_HEAD_-_CYLINDER/IT250E_%281978_MOTORCYCLE%29/CYLINDER_HEAD_-_CYLINDER_%28IT250E_-_1978%29

I always thought running two plugs would make the combustion chamber colder since that is a secondary purpose of a spark plug. So if you do run two plugs maybe you want to run a hotter plug. Something that also never made sense to me is if there is no spark on the back up spark plug wouldn't that plug foul up and be useless anyways. Maybe you could get two coils on that bike and have two working spark plugs. Double the spark means double the power right?? :thumbsup:

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Yeah, the second hole was for a compression release so you could get some engine braking. Many of us put a second plug in because we thought it was cool and we had the mistakenly believed if the first fouled, we could simply move the wire. The truth was eventually the second plug would carbon up and even if it did not, if the first fouled, the second was fouled too.

One of the reasons so few put in the compression release (on top of being broke kids) was as much as it released compression, it also sucked in too. All the unfiltered dirty air. A engine could be ruined in minutes.

The last reason not to is putting a plug there increases the captured CC's of the top end, reducing compression ratio.

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The last reason not to is putting a plug there increases the captured CC's of the top end, reducing compression ratio.

Could you explain this a little more? How does a spark plug differ from the bolt that was originally in the head. They're both spark-plug holes.

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Could you explain this a little more? How does a spark plug differ from the bolt that was originally in the head. They're both spark-plug holes.

The area between the ceramic insulator and the plug shell(the threaded part) is more than what you would have with the blank-off bolt. Therefore, very slightly reduced compression ratio, although I doubt you could actually tell by riding it. The carbon build up from using non synthetic oil would easily compensate for it.

They used to sell compression releases that screwed into that hole. I had one, it even had a one-way valve as part of it to prevent ingesting dirty air. Using it was like throwing out the anchor, which is exactly why Jake Brakes are so popular with truckers. However, being a two stroke, it blew oily goop all over the outside of the head, and by the time the ride was over, your bike had the appearance of someone dumping a quart of used oil on the head for the dirt to stick to. So mine came off after a few weeks of constant washing. It was a real mess.

The other thought was to use two different heat ranges. You would use the hotter range for putt-putting around, then switch over to the colder one for sustained full throttle runs, such as desert riding. I had quite a few Yamahas of that vintage, and the spare hole was filled with the blank-off bolt 99% of the time. Great concept, but not all that useful. Use the hole closest to the carb side of the head, it helps move some heat away from the exhaust port side. That's assuming it has one towards the rear. Some had them on top and then the side. you'll notice none of the modern water cooled bikes have multiple plug holes. It finally died off.

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