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I have a 1980 Yamaha IT 425 that has developed a serious problem that I can't figure out.When I start it, the engine instantly rev's wide open and I have to hit the kill button to shut it down.At first I figured the throttle cable was sticking.Checked it and the slide on the carb. goes up and down ok.Took carb. apart and checked it and the reed valve,everything looked good.Put it back together and the same thing happens.I did notice when it was revving I saw some exhaust coming out the side of the head gasket area.I took the head off (checked the head for flatness and it is not warped)and looked at the gasket,which is solid copper,and noticed it was oily but didnt seem damaged.Could I be getting a bad seal on this gasket and is it possible this is the problem or do you have some other ideas??Could it be C.D.I. related?Also is this a compression gasket that needs to be replaced every time?Thanks.tj bighorn.
 

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I've seen main jets come loose and fall out causing this problem. Have you pulled the float bowl off and checked it? Have you checked to see if the slide needle has popped out? Might be a bad air leak causing it as well.

The IT425 was one hell of a good bike, I bought one brand new in 1980 and put thousands of trouble free miles on it. Those bikes run great!
 
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