Porting and muffler mods

80cc Bicycle Motor Kit

For the most part I followed what others had done here:
MotoredBikes.com Engine Porting - Gases out, Fuel Air in

The piston skirt was cut in a crescent shape as others did, but I decided to match the intake curvature to that. Do this at your own risk! Other people remove material from the bottom of the intake port.

I tried several stones on the Dremel tool to put a slight radius on the ports on the inside to prevent the rings from snagging them. Finally I used a flexible green silicon carbide wheel (about 1" x 1/4") and shaped the edge with the little black brick that came with the set so the wheel edge was round. Worked great.

When I tested it, the lower end torque was dismal, but top end was higher. Not too surprising.

Next I drilled eight 3/16 holes in the bottom plate of the exhaust baffle spaced evenly around the center tube, four 3/16 holes in the upper plate, and some smaller holes in the lower part of the tube below the "pinch". Then I added a generous amount of course steel wool between the two plates. Idle speed is noticeably higher.

I can not really tell that it is any louder, but letting the bike slow to the idle speed, then going WOT gives plenty of acceleration.

My guess is that after porting to get better flow, the gasses need to go somewhere.

I finished the muffler project by putting Locktite on the screw that holds the catalytic converter in, and two new 6-32 screws for the baffle with Nyloc nuts (yes, the Nyloc nuts withstand the temperture). Then I cleaned the outside with acetone, sanded where needed, and sprayed with hi-temp black paint.

The catalytic converter really needs two screws to cut down on the rattle. Next time maybe I will add a sheetmetal screw to the other side.

12-07-08
Slapping the side of the muffler gently with my palm makes quite a rattle from the catalytic converter. A small sheetmetal screw driven in at about 90° to the original screw fixed it entirely.