Hey, I found and old 3.5 lawn mower engine, does anyone know what the best way to rig it up to a bike. Or is it even possible?

I'm sorry, I don't know all the technical terms but I believe that lawnmower engines have to be mounted vertically otherwise it won't work or something. I think the power comes out of a vertical shaft. Motorcycles are driven by a chain and have to be pulled by a horizontally mounted engine. You would have to find some way to transfer the power from vertical to horizontal. Other posters can feel free to correct me. This all comes from my limited go-kart knowledge.

Is The Engine A Vertical Shaft Or A Horizontal Shaft??
If It Is A Horizontal Shaft , It Would Be Much Easier To Do What You Wish To Do.

Check out my 3.5 mower powered bike

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If the engine is an horizontal one, you could easily build a friction drive with a wheel on the top or beside your bike wheel.