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Top end rattle when on throttle (Read 14 times)

Surviving Philly

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Top end rattle when on throttle
Today at 12:25:35

Yo savages,

Having a very concerning rattle when I'm on the throttle, both in motion and when stopped in neutral. No noise when bike is idling -- idles fine, has good compression, runs fine, plug looks fine -- but every time I give it throttle, I hear this rattle (it's quite loud) from the top end of the motor. It stops when I let off throttle.

Any ideas? I have a modified tensioner -- I do remember when my old tensioner was making noise before changing it out, but that noise was a constant, independent of the throttle.

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DragBikeMike

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Re: Top end rattle when on throttle
Reply #1 - Today at 16:43:00

You may have a loose flywheel. A loose flywheel makes a harsh metallic rattle when you throttle up. The loose flywheel problem is fairly common. A number of us have experienced that problem.

I don't think your problem is related to piston slap. Piston slap makes a similar metallic rattle, but it is most predominant at idle and will tend to quiet down a bit when engine speed is raised and when the engine is fully warmed up. But you never can tell. Noises can be difficult to describe, so without having heard the noise in question it's a shot in the dark.

It is also conceivable that you are experiencing detonation. What piston are you running and what octane fuel are you using? If you are using 87 octane fuel, try filling up with 92 octane and see if the noise changes.

This is a video of a loose flywheel. Note how it makes the sharp metallic rattle when I hit the throttle.

https://www.youtube.com/shorts/KUhN_OHQhmo

This is a video of piston slap. Note how the rattle diminishes as I raise engine speed.

https://www.youtube.com/watch?v=Lyt0CWKLCJY

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ohiomoto

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Re: Top end rattle when on throttle
Reply #2 - Today at 17:00:48

Surviving Philly wrote on 08/03/25 at 14:10:50:

With the spring still attached the mechanism rattles against the head and is quite annoying, I also worry about any internal rattling against the rocker arms.

I've experienced no issues without the decomp other than this. I find the decomp unnecessary and have no idea why it's even on the bike... Maybe back when batteries were weaker there was trouble turning the motor over when conditions were right for it (stage of compression, ambient temp) but I consider the decomp solonoid unnecessary and actually problematic -- the main power runs through it bypassing the ignition and it can actually start the motor on its own in some circumstances or will die trying to turn it over until something fails.

------------------

Maybe revisit the decomp delete?

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Surviving Philly Senior Member Offline SuzukiSavage.com Rocks! Posts: 310	Re: Top end rattle when on throttle Reply #3 - Today at 17:49:06 WOW DBM!!!! can't thank you enough I think you are spot on with the flywheel. Very interesting!! As far as your questions, stock piston and I always use the highest octane fuel available.
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