GRiSO 1100 exhaust(?) rattles at 3500 rpm

Hi,

my GRiSO 1100 with original exhaust system has an annoying rattle sound at 3200 - 3600 rpm (maximum at 3500 rpm). It changes a little if the engine is hot or cold. I dismantled the exhaust manifolds, looked at the cover where the monifolds come together but couldn't find the cause. A mechanic at Agostini in Mandello listened to the noise but wasn't either able to locate it. It doesn't seem to be a common problem.
I am sure it is not coming from the last part of the exhaust system (the school cone ) but no evidence.
Has anybody had a similar problem?
I read in the technical description that the exhaust manifolds are double walled design but when I look into them it looks more like single wall - does anybody know if it is single or double wall?
If the manifolds are double wall maybe the inner tube is able to move and at the resonance frequence the vibration turns into the rattle sound.

Any other ideas?

Get yourself a length of metallic tube, copper pipe would do and use it as a stethoscope to locate the source of the noise.
You can buy a engine stethoscope cheaply from ebay if you want. [You must be registered and logged in to see this link.]

Commonest cause for weird exhaust rattles is if the catalyser has come loose. Take it off and give it a shake.

Pete

Hi Pete

This might be a good option to take a look at. It is my first motorcycle with cat - is it located in the central exhaust pipe behind the lambda sensor or in the entrance of the silencer?

I can follow davems hint and put my ear at the cat area. Guess what - I already own the stethoscope shown in link but I hadn't the idea using it for this purpose. I am just afraid that it will be very noisy putting the stethoscope against the silencer at 3500 rpm but it is worth to give it a try.

Thanks a lot

The cat lives in the front of the trombone.

Pete Roper wrote:

The cat lives in the front of the trombone.

Taken out of context, that’s a very odd statement.

And this from a man with a GRiSO with a pink frame!

Hi Pete,

sorry for asking again - trombone means silencer? I am playing tuba and our trombone players don't use catalysators!?
Sometimes I struggle with the slang used here - my english isn't well enough to understand all and everything, especially the hidden jokes  
There is still something to learn

Yes, the huge muffler.

Hi, yesterday I took away the muffler to see if the catalysator is jumping around inside - it wasn't, it was well fixed. But what I saw when I inspected the muffler hanging in my workshop: the stainless steel cover was bended to an oval touching the inner black part. I tried to bend it back into the round shape but without success. I didn't want to remove it completly - it is fixed in a way that it will be complicated to mount it position once it has be taken away.
So I just prepared 2 little sticks of wood and clamped them between the cover and the inner part.
The test ride was a success. After 5 km I have lost one piece of wood and the rattle came back. I repositioned the remaining stick and the rattle was off. Much better now
I will put same oven sealing fabric between muffler and cover we use at work for oven doors. That will not burn away or start fouling. Did anyone ever removed the cover and fixed it again?

Thanks for your help

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Wooden sticks? Oven sealing fabric? This post needs pictures! Log onto the site with your smartphone and you can upload pictures using the "host an image" button. We wait...

I tried to add some pics but the links were eaten by the system. I have to try again with firefox, maybe opera is not well supported. I have observed some difficulties using opera the last days.
I travel to China for 2 weeks, then I will organize the anti-rattling material. When the work is done I will write a report.

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The catalysator is in the first part of the muffler

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The cover touches the inner part

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A little piece of wood stops the rattle...

It looks like it works this time to add the pics.