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Any engine will be noticeably louder at wider throttle settings, with air filter mods that increase flow. Even a K&N type filter, will raise noise levels, over an OEM setup, when throttle is opened to pull a hill and such. This is normal......the drop in power is not, and I don't know what would cause a hissing....seems like something would have to be pressurized and leaking a bit to cause that.
Runs alright at moderate throttle, spark plug shows no color, looses power at wider throttle settings....all adds up to lean....too much air for available fuel. Don't fiddle with ignition system.
One question, have you done anything to the emmissions system on this scoot?
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