does anyone know how to build a custom exhaust for a weedeater?

i am trying to build a motor bike and i want to supe up a 25cc craftsman trimmer motor the bike will be spindle drive and i want to suck any bit of juice out of that engine... once i am done i will let you know how it went

You would have to play with making different size expansion chambers (the fat part like on 2 stroke dirtbikes). There is actually quite the science behind it. Here is a website on it http://www.motorcycle.com/how-to/how-twostroke-expansion-chambers-work-and-why-you-should-care-3423.html and also on another note, just so you know string trimmer (weed eater) engines and other engines like it ( blowers, chainsaws ) are ment to be run at full throttle. The carburators have a low speed circut and a high speed circut . (unlike motorbikes that have a way to deliver the correct amount of fuel in al throttle positions) People tend to run into trouble when they are running them at a mid range and find that they end up with scoring across the exhaust side of the piston. Not trying to discourage you, but also wouldn't want to see someone make something awesome and then burn it out in a week.