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I lined the interior of my engine cowling with Dynamite XT (http://www.dynamat.com/automotive-and...at-xtreme/). It made BIG difference - quieted the engine down quite a bit. I added pictures of the installation to my personal page.

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Glad it worked for you. I've seen this stuff before and thought it pretty cool.

A couple of questions if you don't mind: Who did you purchase it from, and approx. what did it cost? I would have assumed there was some sort of sound proofing in the cowl when the motor was made. Did you remove it and replace it or was it missing when you got the motor? When you say it quieted it down, is that compared to the original sound insulation or with nothing at all under there?

Thanks!

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I purchased on Amazon and the cost for the whole job was about $45. My motor had no factory installed sound proofing material. I took sound meter measurements near the cowling before and after and the reduction was about 3db which is a halfing of the sound level.

It's not as quiet as a 4 stroke but much better. The best way to describe the change is that at cruizing speed (4200rpm and 28mph) the engine noise was uncomfortable and it was hard to hold a conversation with a person sitting next to you. Now the engine noise is not uncomfortable, the predominant noise is wind noise, and one can comfortably hold a conversation.

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Nice looking rig! Curious the motor had nothing in there before.

Where in NC are are you located? I have a beach house in Corolla, NC on the north end of the Outer Banks...

In any event I have 1999 135 Optimax with the original sound proofing on the inside of the cowl. The one complaint I have with the motor is that is LOUD. It's already a tight fit around the motor with the factory sound proofing, but I wonder if the dynamat would be better or if it could be used in conjunction with the existing foam insulation.

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Corolla - Very nice! I'm in Raleigh but often make it out to the NC beaches during the summer months.

The idea behind Dynamat is that it has mass but little to no elasticity so it absorbs the sound. It needs to be in direct contact with the surface you are trying to silence to be most effective. If you were to try it you would want to remove the original sound proofing first, install the dynamite directly on the inside of the cowling and then perhaps place the factory sound proofing on top of the Dynamat. If removing the factory sound proofing will destroy it I would be hesitant ...