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Thread: J-max not hitting rpm's

  1. #1
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    Default J-max not hitting rpm's

    I have an extreme shallow jetcraft with a yamaha jmax engine. All of the sudden it wont get up to the proper rpm's, has anyone ever had this happen. Is it a bad plug??????????

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    I had a similar problem with a single cylinder motorcycle (Yamaha). Check the spark plug torque. If it is too loose you will not get a good ground and will have a weak spark along with the possibility of not getting full compression. My plug was barely past hand tight, I torqued it and the problem went away. Good luck.

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    I pulled the plugs today and they were all tight, I did notice that 3 of them looked the same color and one plug was a quite abit darker than the others, so Im guessing its a bad plug. It idles just fine, no miss, but once I get on it it seems to run fine but wont get up to full rpm. I ordered new plugs this morning. When they get here I am going to put them in and try it.

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    Default let us know

    esp. the details of what you have (engine specs. inluding year) and what you find vs. what you've done and what happens after a change. Maybe you should contact Yamaha on this. I'm paying attention... keep me in the loop. Good luck bro.

    p.s. did you check the spark plug gap/s? How sure enough good is your fuel? And especially, how new is your engine and what maintenance has been done?

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    I figured it out, I put new plugs in it and it works great. This engine only has about 100 hours on it and I change the oil twice a year. The darker looking plug must not have been firing enough when at wide open throttle. Shes hittin 9800 rpm now.

  6. #6
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    Quote Originally Posted by rhino36 View Post
    I figured it out, I put new plugs in it and it works great. This engine only has about 100 hours on it and I change the oil twice a year. The darker looking plug must not have been firing enough when at wide open throttle. Shes hittin 9800 rpm now.
    Wow, almost 10K rpm. What's redline in that thing?

  7. #7
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    10K is what its suspose to run at, Im 200 rpms low but thats close enough.

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    Glad you got her figured out. I know how frustrating that kind of thing can be.

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