big bore kit?


Well-Known Member
Dec 23, 2007
Kalamazoo, MI
from my understanding the bicycle motor has a lining that makes it next to impossible to bore out the cylinder


Active Member
Jan 27, 2008
Ptown, Texas
Norman (aka Lord Vader) discovered when he tore his engine down that his wrist pin bushing was drilled off center which essentially made the stroke a little longer than stock. His bicycle will fly. So that might be something you could try.......................

Jemma Hawtrey

New Member
Dec 29, 2007
Essex, UK
I think I remember someone doing something similar regarding austin 7 based racing which I found on some site which was basically a write up of archive from the '60s. The motor concerned is a 10hp rated 747cc side valve (or in americanese, flathead) 4 cylinder.

The only problem was if the drilling was too off centre things parted company and large clouds of smoke and much havoc ensued.

There was also an unusual review of an old (1911 or so) racing car with a huge motor and an exhaust twist. There was a remotely operated valve half way between the manifold and the primary silencer that opened the exhaust to atmosphere when it was operated and apparently gave a useful hike in power.. something for the 4-strokers to ponder maybe?

jemma xx

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
up north now
Smaller rear sprocket=More speed.

You can also open up your exhaust (cut or remove the baffle) clean up the intake and exhaust ports, install a thinner base gasket and a lot of other things for a little more power.

More power=less reliability, though when done right, I think you could get a modest power boost without too much sacrifice.

Also remember when making "adjustments" to a two stroke bicycle motor, to keep an eye on the jetting so you don't fry the engine.